Adding right err message instead of exit code for the git pull-request --fetch

This pull request makes gh more verbose by copying the stderr (standard error) to the cli instead of simply failing silently.

For example:
```
$ gh pr -n 2 -u eduardolundgren --fetch
gh [info] Fetching pull request #2 into branch pr-2
gh [error] fatal: Refusing to fetch into current branch refs/heads/pr-2 of non-bare repository
fatal: The remote end hung up unexpectedly
```

instead of

```
$ gh pr -n 2 -u eduardolundgren --fetch
gh [info] Fetching pull request #2 into branch pr-2
gh [error] 128
```