Return more verbose error message

Hi,

Right now if the network device returns an error, it's properly logged in the logfile, while only "Unable to make changes" is returned to Ansible. To me, the most verbose/useful error should be returned to Ansible.
If so, I think nothing gets caught in the "except CommitError" and defaults to "except Exception".
I'll wait before trying to send a pull request :P

Thx!
