Fall back to loading commit authors in UserStore

[smashwilson]

Prerequisites

• Have you tried to reproduce the problem in Safe Mode?
• Have you followed all applicable steps in the debugging guide?
• Have you checked the FAQs on the message board for common solutions?
• Have you checked that your issue isn't already filed?

Description

When the Repository has a GitHub remote, but GithubLoginModel fails to retrieve mentionable users from the GraphQL API for any reason (repository deleted; unauthenticated; bad token; network error) the UserStore is left empty. As a result, the co-author drop-down is not populated.

Steps to Reproduce

1. Create a git repository and make a few commits. Add a remote that looks like a GitHub URL, but for a repository that doesn't exist; git remote add origin git@github.com:atom/nah.git. Open it in Atom.
2. Open the Git tab and expand the co-author input.
3. Type the first few letters of your git username in the co-author input.

Expected behavior:

The autocomplete list should appear and show your git username and email address.

Actual behavior:

The autocomplete list is empty.

Reproduces how often:

100%

Versions

macOS Mojave 10.14.2

$ atom --version
Atom    : 1.35.0-nightly11
Electron: 2.0.16
Chrome  : 61.0.3163.100
Node    : 8.9.3
$ apm --version
apm  2.1.3
npm  6.2.0
node 8.9.3 x64
atom 1.35.0-nightly11
python 2.7.15
git 2.20.1

Additional Information

N/A