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

[johnjbarton]

Fixes #3296

[kyliau]

the logic above this line assumes that message is a string, so perhaps it's better to do the following:

    var message = messageOrEvent
    if (typeof message === "string") {
        var location = getLocation(source, lineno, colno)
        if (location !== '') {
          message += '\nat ' + location
        }
        if (error) {
          message += '\n\n' + error.stack
        }
     }
     else {
      // 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()}
    }

[johnjbarton]

Please take another look (the appveyor builds passed but failed to update github)