Friendlier messages

When executing gush on a directory that is not a git repository we get a PHP Fatal error as result:

`PHP Fatal error:  Uncaught exception 'RuntimeException' with message 'fatal: Not a git repository (or any of the parent directories): .git' in phar:///usr/local/bin/gush/src/Helper/ProcessHelper.php:81`

I find a better approach would be to simply show a message like "Gush works only inside a git repository."