Assert: Report RegExp/Error as strings from rejects()/throws()

Report the `actual` (Error object) as a string.
And report an `expected` RegExp or Error object also in its
string form.

Currently, they were reported as their objects, which in the
generic js-reporters module was just printed as an empty object
without any properties because instances of RegExp and instances
of Error both have no own properties that are enumerable.

Also fix errorString() which oddly had logic for passing a plain
object, but didn't

Fixes #1333.

src/assert.js

@@ -286,12 +286,14 @@ class Assert {
 			// We don't want to validate thrown error
 			if ( !expected ) {
 				result = true;
-				expected = null;
 
 			// Expected is a regexp
 			} else if ( expectedType === "regexp" ) {
 				result = expected.test( errorString( actual ) );
 
+				// Log the string form of the regexp
+				expected = String( expected );
+
 			// Expected is a constructor, maybe an Error constructor
 			} else if ( expectedType === "function" && actual instanceof expected ) {
 				result = true;
@@ -302,6 +304,9 @@ class Assert {
 					actual.name === expected.name &&
 					actual.message === expected.message;
 
+				// Log the string form of the Error object
+				expected = errorString( expected );
+
 			// Expected is a validation function which returns true if validation passed
 			} else if ( expectedType === "function" && expected.call( {}, actual ) === true ) {
 				expected = null;
@@ -311,7 +316,9 @@ class Assert {
 
 		currentTest.assert.pushResult( {
 			result,
-			actual,
+
+			// undefined if it didn't throw
+			actual: actual && errorString( actual ),
 			expected,
 			message
 		} );
@@ -378,12 +385,14 @@ class Assert {
 				// We don't want to validate
 				if ( expected === undefined ) {
 					result = true;
-					expected = actual;
 
 					// Expected is a regexp
 				} else if ( expectedType === "regexp" ) {
 					result = expected.test( errorString( actual ) );
 
+					// Log the string form of the regexp
+					expected = String( expected );
+
 					// Expected is a constructor, maybe an Error constructor
 				} else if ( expectedType === "function" && actual instanceof expected ) {
 					result = true;
@@ -394,6 +403,9 @@ class Assert {
 						actual.name === expected.name &&
 						actual.message === expected.message;
 
+					// Log the string form of the Error object
+					expected = errorString( expected );
+
 					// Expected is a validation function which returns true if validation passed
 				} else {
 					if ( expectedType === "function" ) {
@@ -412,7 +424,9 @@ class Assert {
 
 				currentTest.assert.pushResult( {
 					result,
-					actual,
+
+					// leave rejection value of undefined as-is
+					actual: actual && errorString( actual ),
 					expected,
 					message
 				} );
@@ -431,12 +445,14 @@ Assert.prototype.raises = Assert.prototype[ "throws" ];
 /**
  * Converts an error into a simple string for comparisons.
  *
- * @param {Error} error
+ * @param {Error|Object} error
  * @return {String}
  */
 function errorString( error ) {
 	const resultErrorString = error.toString();
 
+	// If the error wasn't a subclass of Error but something like
+	// an object literal with name and message properties...
 	if ( resultErrorString.substring( 0, 7 ) === "[object" ) {
 		const name = error.name ? error.name.toString() : "Error";
 		const message = error.message ? error.message.toString() : "";

test/cli/fixtures/expected/tap-outputs.js

@@ -54,12 +54,12 @@ ok 2 Second > 1
 
 	"qunit fail/throws-match.js":
 `TAP version 13
-not ok 1 global failure
+not ok 1 Throws match > bad
   ---
   message: "match error"
   severity: failed
-  actual: {}
-  expected: {}
+  actual: "Error: Match me with a pattern"
+  expected: "/incorrect pattern/"
   stack: .*
   ...
 1..1

test/main/assert.js

@@ -252,7 +252,7 @@ QUnit.test( "throws", function( assert ) {
 			};
 		},
 		{ name: "SomeName", message: "some message" },
-		"thrown error object is similar to the expected plain object"
+		"thrown object is similar to the expected plain object"
 	);
 
 	assert.throws(
@@ -378,7 +378,7 @@ QUnit.test( "rejects", function( assert ) {
 			message: "some message"
 		} ),
 		{ name: "SomeName", message: "some message" },
-		"thrown error object is similar to the expected plain object"
+		"thrown object is similar to the expected plain object"
 	);
 
 	assert.rejects(