introduce global error message handling

Motivation:

A meaningful error messages is very helpful for client. It can clearly show what a client is doing wrong and provides information that can help to diagnose the problem.

Although, spring come with a generic error message but this however is too verbrose and unneccessarily reveals technical details.

Modification:

• create a custom error response class
• use the global exception handler mechanism
• update both the documentation and the api handlers to throw appropriate exceptions
• create a generic entity-not-found exception

Result:

A good starting point in creating a roboust error handling system in cerebrum. Now, we have a bear minimum in error handling.

Target: master Acked-by: Merge-request: Committed: master@01f82283c4764299b61f0a1440d952bc2e1be71d