handling action rejection, server timeout

[Spir0u]

Public API Changes
Actions forward an error when not available/they get cancelled

Description

1. When no server is available, I raise an Eexception to get feedback.
2. throwing an exception in goal_response_cb does not work properly, therefore I return it in the self.result. Probably there would be a better option, but this was working for me.
3. The result was still none when the action was cancelled, so I put the empty result directly in the return of execute_callback.