Hitting GitHub API rate limit clears received PR lists

[davkal]

• Extension version: 0.1.4
• VSCode Version: 1.27.1
• OS: Mac

Steps to Reproduce:

1. Open Code on a Grafana clone
2. Expand Github Pull Requests (syncs fine, PR numbers show up)
3. Expand All PRs list
4. Click on a PR, and select some files.

Result:

• a rate limit warning is displayed.
  

• the PR lists are cleared.

Expected:

• keeping the lists
• even better: a way to get around the rate limit

[RMacfarlane]

I just checked in a change that should make it much less likely for the API rate limit to be hit: 7a58197

Let me know if you're still seeing this in version 0.1.5 of the extension

[davkal]

The lists are no longer cleared, but I still get the limit warning sometimes.
Feel free to close the issue unless you want to use it for the rate limit itself.

[rebornix]

Same issue as #745