fix(builder): disallow password authentication

Closes #1340

Replaces the previous password prompt with an error message similar to:

```console
kamina ~/tmp/deis/example-go (git:master) ✔
$ git push deis master
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

Ideally we would brand this with a Deis-centric message, but for now
this should be good enough until we overhaul the builder completely.