better error message for push failures

When a remote is ahead of a local branch, dolt push errors out with this message:

`can't fast forward merge`

Git is more helpful:

```
error: failed to push some refs to 'https://.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again
```