Don't fail on requesting reviewers

[joneshf]

It's not always possible to request reviewers, so we don't always fail.

Unless you have write access to a repo, you cannot request reviewers for
a PR. Since there are workflows where not everyone has write access, and
we don't want to fail unnecessarily, we don't make it a requirement that
requesting reviewers has to succeed.

Test Plan:

1. First, ensure you don't have write access to a repo.
2. Create a commit.
3. Run spr diff
4. The PR should be opened and the command should only warn when it
   cannot request reviewers.

[joneshf]

Sorry, opened too soon. Lemme try again.