fix: use Exceptions instead of Errors where appropriate

lib/src/core/consumed_thing.dart

@@ -36,6 +36,20 @@ class UnexpectedReponseException implements Exception {
   }
 }
 
+/// This Exception is thrown when
+class SubscriptionException implements Exception {
+  /// The error [message].
+  final String message;
+
+  /// Creates a new [SubscriptionException] from an error [message].
+  SubscriptionException(this.message);
+
+  @override
+  String toString() {
+    return message;
+  }
+}
+
 /// Implementation of the [scripting_api.ConsumedThing] interface.
 class ConsumedThing implements scripting_api.ConsumedThing {
   /// The [Servient] corresponding with this [ConsumedThing].
@@ -294,7 +308,8 @@ class ConsumedThing implements scripting_api.ConsumedThing {
     }
 
     if (_subscribedEvents.containsKey(eventName)) {
-      throw ArgumentError("ConsumedThing '$title' already has a function "
+      throw SubscriptionException(
+          "ConsumedThing '$title' already has a function "
           "subscribed to $eventName. You can only subscribe once.");
     }
 

lib/src/core/servient.dart

@@ -17,6 +17,17 @@ import 'protocol_interfaces/protocol_server.dart';
 import 'security_provider.dart';
 import 'wot.dart';
 
+/// Exception that is thrown by a [Servient].
+class ServientException implements Exception {
+  final String _message;
+
+  /// Constructor
+  ServientException(this._message);
+
+  @override
+  String toString() => "$runtimeType: $_message";
+}
+
 // TODO(JKRhb): Documentation should be improved.
 /// A software stack that implements the WoT building blocks.
 ///
@@ -194,7 +205,8 @@ class Servient {
     if (hasClientFor(scheme)) {
       return _clientFactories[scheme]!.createClient(_clientSecurityProvider);
     } else {
-      throw StateError('Servient has no ClientFactory for scheme $scheme');
+      throw ServientException(
+          'Servient has no ClientFactory for scheme $scheme');
     }
   }
 }

lib/src/core/wot.dart

@@ -27,6 +27,22 @@ class ThingConsumptionException implements Exception {
   }
 }
 
+/// This [Exception] is thrown if an error during the production of a
+/// [ThingDescription] occurs.
+class ThingProductionException implements Exception {
+  /// The identifier of the [ThingDescription] that triggered this [Exception].
+  final String identifier;
+
+  /// Constructor
+  ThingProductionException(this.identifier);
+
+  @override
+  String toString() {
+    return "$runtimeType: An ExposedThing with identifier $identifier already "
+        "exists.";
+  }
+}
+
 /// Implementation of the [scripting_api.WoT] runtime interface.
 class WoT implements scripting_api.WoT {
   final Servient _servient;
@@ -59,7 +75,8 @@ class WoT implements scripting_api.WoT {
     if (_servient.addThing(newThing)) {
       return newThing;
     } else {
-      throw StateError('Thing already exists: ${newThing.title}');
+      final id = newThing.thingDescription.identifier;
+      throw ThingProductionException(id);
     }
   }
 

lib/src/definitions/expected_response.dart

@@ -4,6 +4,8 @@
 //
 // SPDX-License-Identifier: BSD-3-Clause
 
+import 'validation/validation_exception.dart';
+
 /// Communication metadata describing the expected response message for the
 /// primary response.
 class ExpectedResponse {
@@ -18,7 +20,7 @@ class ExpectedResponse {
 
   static String _parseContentType(dynamic contentType) {
     if (contentType is! String) {
-      throw ArgumentError("contentType of response map is not a String!");
+      throw ValidationException("contentType of response map is not a String!");
     }
     return contentType;
   }

lib/src/definitions/form.dart

@@ -385,7 +385,7 @@ class Form {
     }
 
     if (uriVariables == null) {
-      throw ArgumentError("The Form href $href contains URI variables "
+      throw ValidationException("The Form href $href contains URI variables "
           "but no values were provided as InteractionOptions.");
     }
 

lib/src/definitions/link.dart

@@ -4,6 +4,8 @@
 //
 // SPDX-License-Identifier: BSD-3-Clause
 
+import 'validation/validation_exception.dart';
+
 /// Represents an element of the `links` array in a Thing Description.
 ///
 /// A link can be viewed as a statement of the form "link context has a relation
@@ -67,7 +69,7 @@ class Link {
       href = Uri.parse(hrefString);
     } else {
       // [href] *must* be initialized.
-      throw ArgumentError("'href' field must exist as a string.", "formJson");
+      throw ValidationException("'href' field must exist as a string.");
     }
 
     if (json["type"] is String) {

lib/src/definitions/security/oauth2_security_scheme.dart

@@ -4,6 +4,7 @@
 //
 // SPDX-License-Identifier: BSD-3-Clause
 
+import '../validation/validation_exception.dart';
 import 'helper_functions.dart';
 import 'security_scheme.dart';
 
@@ -92,7 +93,7 @@ class OAuth2SecurityScheme extends SecurityScheme {
       flow = jsonFlow;
       _parsedJsonFields.add("flow");
     } else {
-      throw ArgumentError("flow must be of type 'string'!");
+      throw ValidationException("flow must be of type 'string'!");
     }
 
     parseAdditionalFields(additionalFields, json, _parsedJsonFields);

lib/src/definitions/thing_description.dart

@@ -23,6 +23,7 @@ import 'security/psk_security_scheme.dart';
 import 'security/security_scheme.dart';
 import 'thing_model.dart';
 import 'validation/thing_description_schema.dart';
+import 'validation/validation_exception.dart';
 
 const _validContextValues = [
   "https://www.w3.org/2019/wot/td/v1",
@@ -196,7 +197,7 @@ class ThingDescription {
     if (titleJson is String) {
       title = titleJson;
     } else {
-      throw ArgumentError("Thing Description type is not a "
+      throw ValidationException("Thing Description type is not a "
           "String but ${title.runtimeType}");
     }
   }

lib/src/scripting_api/subscription.dart

@@ -9,6 +9,17 @@ import '../definitions/interaction_affordances/interaction_affordance.dart';
 import '../definitions/operation_type.dart';
 import 'interaction_options.dart';
 
+/// [Exception] that is thrown when error during the unsubscribe process occurs.
+class UnsubscribeException implements Exception {
+  final String _message;
+
+  /// Constructor.
+  UnsubscribeException(this._message);
+
+  @override
+  String toString() => "$runtimeType: $_message";
+}
+
 /// Indicates the type of the subscription.
 enum SubscriptionType {
   /// The subscription is the observation of a property.
@@ -61,7 +72,7 @@ Form findUnsubscribeForm(InteractionAffordance interaction,
   final unsubscribeForm = _findFormByScoring(interaction, form, operationType);
 
   if (unsubscribeForm == null) {
-    throw StateError("Could not find matching form for unsubscribe");
+    throw UnsubscribeException("Could not find matching form for unsubscribe");
   }
 
   return unsubscribeForm;