Allow setting of custom error tracking handler

[the-overengineer]

• Add support for custom error handler for JS errors, called with (event, eventName?)
• Document changes
• Add test

[coveralls]

Coverage remained the same at 100.0% when pulling caaba7bce097aebf4cd4242471a936ffb699191f on oradian:master into 999fe4b450818393d375e13dce418230c31483f5 on joernroeder:master.