Improve error message if something fails executing the git receiver in a push

[aledbf]

Right now if there is an error executing /app/templates/receiver during a push I only see something like

To ssh://git@deis.deis.io:2222/app-demo.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'ssh://git@deis.deis.io:2222/app-demo.git'

There is no indication of what's wrong.
The problem is that the curl inside is using --silent and there is no check after the execution and if the curl status code return other thing than 200 it will return an error but it will not show the reason.

[mboersma]

Closing this as a duplicate of #1772 (git push errors could be more helpful than "permission denied").