Robust Error handling

The current code is rather weak on error handling: return codes are often ignored, errors are logged but not handled or might lead right away to a panic.

related issues #81, #87, #104, #139

We probably should come up first with a general error strategy of how to deal errors in enclaves and pass them on and alike and the i guess slowly chip away on it. What we definitely should do is start at least adding TODOs of code we touch related to this so at least we know what still have to be done ...

(For fabric general error handling guidelines, see https://hyperledger-fabric.readthedocs.io/en/latest/error-handling.html)