More descriptive error messages (#108)

* more descriptive error messages

* fix typos

lib/discard/model.rb

@@ -159,11 +159,23 @@ def undiscard!
     private
 
     def _raise_record_not_discarded
-      raise ::Discard::RecordNotDiscarded.new("Failed to discard the record", self)
+      raise ::Discard::RecordNotDiscarded.new(discarded_fail_message, self)
     end
 
     def _raise_record_not_undiscarded
-      raise ::Discard::RecordNotUndiscarded.new("Failed to undiscard the record", self)
+      raise ::Discard::RecordNotUndiscarded.new(undiscarded_fail_message, self)
+    end
+
+    def discarded_fail_message
+      return "A discarded record cannot be discarded" if discarded?
+
+      "Failed to discard the record"
+    end
+
+    def undiscarded_fail_message
+      return "An undiscarded record cannot be undiscarded" if undiscarded?
+
+      "Failed to undiscard the record"
     end
   end
 end

spec/discard/model_spec.rb

@@ -95,7 +95,7 @@
         it "raises Discard::RecordNotUndiscarded" do
           expect {
             post.undiscard!
-          }.to raise_error(Discard::RecordNotUndiscarded)
+          }.to raise_error(Discard::RecordNotUndiscarded, 'An undiscarded record cannot be undiscarded')
         end
       end
     end
@@ -153,7 +153,7 @@
         it "raises Discard::RecordNotDiscarded" do
           expect {
             post.discard!
-          }.to raise_error(Discard::RecordNotDiscarded)
+          }.to raise_error(Discard::RecordNotDiscarded, "A discarded record cannot be discarded")
         end
       end
 
@@ -616,7 +616,7 @@ def abort_callback
           expect(post).to receive(:do_before_discard) { abort_callback }
           expect {
             post.discard!
-          }.to raise_error(Discard::RecordNotDiscarded)
+          }.to raise_error(Discard::RecordNotDiscarded, "Failed to discard the record")
         end
       end
     end
@@ -677,11 +677,11 @@ def abort_callback
       end
 
       describe '#undiscard!' do
-        it "raises Discard::RecordNotDiscarded" do
+        it "raises Discard::RecordNotUndiscarded" do
           expect(post).to receive(:do_before_undiscard) { abort_callback }
           expect {
             post.undiscard!
-          }.to raise_error(Discard::RecordNotUndiscarded)
+          }.to raise_error(Discard::RecordNotUndiscarded, "Failed to undiscard the record")
         end
       end
     end