extension host: Permanent HTTPStatusError after transient error

Created by: efritz

Steps to reproduce:

1. Start enterprise instance
2. Navigate to a blob view (I don't think this code needs a precise index to replicate)
3. Hover over some things and get those sweet, sweet hover messages
4. Restart your instance
5. Hover over things while instance is down, note the console messages and missing results
6. Hover over things after instance comes back up, note continued console messages and missing results

Additional notes:

• If we open the same blob view in a new tab (or refresh) it becomes functional again
• If we navigate away from that document, the new document works. If we navigate back to the old document, it's still broken.
• We see these error messages in the console:

Expected behavior:

The extension should be able to access the Sourcegraph instance API after a transient network error.

Actual behavior:

It isn't.