[ENHANCEMENT] `heroku git:remote` fails with unhelpful message when not a git repository

[justinwilaby]

When a user executes heroku git:remote --app APP_NAME in the wrong cwd, the CLI fails with a "not a github repository". This error message does not provide enough details on how to remedy or correct the error. This enhancement is to add better, more clear messaging.

Something like: " is not a git remote. Please re-run this command in a git initialized directory"