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

[henvic]

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

[zenorocha]

Is this related to #126?

[zenorocha]

Just started reviewing :)

Sent from GH.

[zenorocha]

It worked! Thanks @henvic :)

P.S.: To simulate this issue you need to fetch a PR into an existing local branch.

[zenorocha]

Thank you, pull request merged!

Sent from GH.