GitHub Importer - Notify user when rate limit is reached

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Problem

As described in #415125 (closed), when GitHub's API limit is reached, the import fails silently and does not present errors to the user.

Proposal

Let's notify the user in the UI when the rate limit is reached:

• When the rate limit is reached backend needs to pass this information on to frontend
• frontend will display a warning/error message in the UI.