Make it possible to hide error notifications

[samwho]

Hi there! Really like this extension, but quite frequently the request to fetch notifications will fail and I get an error notification popup.

I'd prefer not to see these but there's no way to disable them. Even the new ability to hide notifications per-extension doesn't work because they're errors, and errors are always shown.

The code for it appears to be here:

github-notifications/src/extension.ts

Line 208 in cb1cd20

error && vscode.window.showErrorMessage(message);

I can think of a few options:

1. Implement some kind of retry mechanism on the fetch on line 267 that would prevent an error being thrown.
2. Give users an option to not send an error notification for this fetch.
3. Downgrade this from an error notification to a warning so it can be hidden by the VSCode "do not disturb" functionality.

Thanks again for making this extension, it's incredibly underrated!