forward operation result in finished signal

Thinking about, that this doesn't really hurt ... This allows us to review the result of an operation and give the user a helpful hint on what went wrong and what to do. The error messages are very close to the Git commandline.

The alternate is to keep a member of an (asynchronus) operation and request the result within the `finished()` slot. But we can do that still either. This is just an easy way :sun_with_face: