consistent error determination

The child processes determine whether the test had an error based on its `error`
property not being `undefined`. However they then change this property to an
empty object if there was no error.

The API determines whether a test had an error based on its (empty) error object
having a `message` property. If a test did not have an error, its `error`
property is set to `null`. This prevents errors without messages from being
reported correctly.

Instead set `error` to `null` in the child processes and rely on that in the
API.

I've added a test to validate errors without messages get reported.

api.js

@@ -114,9 +114,7 @@ Api.prototype._handleStats = function (stats) {
 Api.prototype._handleTest = function (test) {
 	test.title = this._prefixTitle(test.file) + test.title;
 
-	var isError = test.error.message;
-
-	if (isError) {
+	if (test.error) {
 		if (test.error.powerAssertContext) {
 			var message = formatter(test.error.powerAssertContext);
 
@@ -132,8 +130,6 @@ Api.prototype._handleTest = function (test) {
 		}
 
 		this.errors.push(test);
-	} else {
-		test.error = null;
 	}
 
 	this.emit('test', test);

index.js

@@ -46,11 +46,14 @@ function test(props) {
 		return;
 	}
 
-	props.error = hasError ? serializeError(props.error) : {};
-
+	if (hasError) {
+		props.error = serializeError(props.error);
 		if (props.error.stack) {
 			props.error.stack = beautifyStack(props.error.stack);
 		}
+	} else {
+		props.error = null;
+	}
 
 	send('test', props);
 

test/api.js

@@ -279,6 +279,17 @@ test('uncaught exception will throw an error', function (t) {
 		});
 });
 
+test('errors can occur without messages', function (t) {
+	t.plan(2);
+
+	var api = new Api([path.join(__dirname, 'fixture/error-without-message.js')]);
+	api.run()
+		.then(function () {
+			t.is(api.failCount, 1);
+			t.is(api.errors.length, 1);
+		});
+});
+
 test('stack traces for exceptions are corrected using a source map file', function (t) {
 	t.plan(4);
 

test/fixture/error-without-message.js

@@ -0,0 +1,5 @@
+import test from '../../';
+
+test('throw an error without a message', () => {
+	throw new Error();
+});

test/hooks.js

@@ -309,7 +309,7 @@ test('don\'t display hook title if it did not fail', function (t) {
 	fork(path.join(__dirname, 'fixture', 'hooks-passing.js'))
 		.run()
 		.on('test', function (test) {
-			t.same(test.error, {});
+			t.same(test.error, null);
 			t.is(test.title, 'pass');
 		})
 		.then(function () {