Make default ExceptionHandler logging message more informative

[tkroman]

I've seen people wondering about the messages logged from the default ExceptionHandler more than once, especially when exception's message is null (we get something like this:

Error during processing of request: 'null'. Completing with InternalServerError response

which is not very helpful.)

Since there's no stack trace logging, I assume it's due to performance considerations and don't suggest simply adding an exception object as a log.error argument, but rather

a) printing a class name for exceptions with e.getMessage == null
b) log message with a link to the 'Exception Handling' section of the docs and a short error description.

I've tried to do this in a local branch, so if you think this is worth a PR or further discussion, here's the change I propose.

[jrudolph]

Thanks, @tkroman. I like this suggestion. I wonder if it could even make sense to log the exceptions as exceptions so that the logger can directly log the stack traces.

Please open a PR with your suggestion for further discussion.