fix(client): Only create the funky object if message is not a string

Fixes karma-runner#3296

client/karma.js

@@ -141,9 +141,11 @@ function Karma (socket, iframe, opener, navigator, location) {
       message += '\n\n' + error.stack
     }
 
-    // create an object with the string representation of the message to ensure all its content is properly
-    // transferred to the console log
-    message = {message: message, str: message.toString()}
+    if (typeof message !== 'string') {
+      // create an object with the string representation of the message to ensure all its content is properly
+      // transferred to the console log
+      message = {message: message, str: message.toString()}
+    }
 
     socket.emit('karma_error', message)
     this.complete()