fail on invalid push

If the assumed or provided remote doesn't exist, The script should bail. Currently I'm seeing:

```
§ pullme
Do you want to push to alorente/master? [Ynaq?] 
fatal: 'alorente' does not appear to be a git repository
fatal: The remote end hung up unexpectedly
looking for an appropriate base branch
Do you want to submit a pull request to origin/master? [Ynaq?] q
```
