Improve error handling when git push fails

If for some reason the user doesn't have the correct SSH credentials for either remote repository, Paratrooper should inform the user that the push has failed for tags or for the deployment itself. I've spiked this with some working code, but the notifications are incomplete, i.e. it does what it's supposed to do, but it doesn't tell the user.

I would probably define a new exception class for each failure and raise the exception based on the value returned by the push command.