More friendly message when pushing to the read-only url

Github gives a nice warning when a user tries to push to the read-only git url:

fatal: remote error: 
  You can't push to git://github.com/REPO/foo.git
  Use git@github.com:REPO/foo.git

the system on d.o. currently just outputs this:

fatal: The remote end hung up unexpectedly

leading one to wonder if the remote server is down, rather than the quick fix this actually is.

There is a related issue here: #1050892: ssh: connect to host git-dev.drupal.org port 2222: Connection timed out fatal: The remote end hung up unexpectedly.