git push errors could be more helpful than "permission denied"

_From @ianblenke on September 4, 2014 20:33_

This is a reoccurring problem for everyone. For various reasons, either no ssh key is available for the push that matches a user (the user forgot to add their key, their key isn't on their agent keyring, etc), or the ssh-agent presents a key for another user that doesn't have access to push to an app.

While it is fine to talk people through "ssh-add -l" and "ssh-add -D" and "ssh-add ~/.ssh/id_rsa-deis", that's just the first layer. The second layer is telling people to use "deis keys:list" and "deis keys:add" to add the key for their user.

It would help to return two possible error messages during a git push that is rejected:

```
No user was found for any of your presented keys. Please try "deis keys:list" and "deis keys:add"
```

and

```
You presented a key for jsmith, but that user does not have shared permissions for the app you are pushing to.
```

These two things would help users realize what is going on and maybe take action themselves without having to resort to asking for support.

_Copied from original issue: deis/deis#1772_