Add meaningful response for 500er

[dirk-ecker]

Actual the response of a 500er contains useless data:

code: 500
message: "Server Error"

The meaning of a 500 is a problem in the server code. Those problems hopefully never get into production. I think for development it would be a good idea to enrich those responses with a stacktrace and more details about what has happend so most of the time you have not to search in the logs or elsewhere.