No way to return an error with arbitrary message

[urmaul]

I have the "catch all panics" wrapper around the code and return a JNI error in this case. With the introduction of errors enum in 0.2.1 this stopped working. Right now JniError::Unknown looks like the best type for this case but there is no way to also provide the error message.

[skaldebane]

Might be related to #25?

[uvlad7]

This can be an implementation example