Native integration treats unauthed HTTP response from auth proxy as error if auth proxy sends no CORS headers

Created by: slimsag

At a large customer instance https://app.hubspot.com/contacts/2762526/company/407948923/ we turned on the Bitbucket Server integration.

They have an auth proxy in front of Sourcegraph which is required for all users to access Sourcegraph, and does not forward any CORS headers from the application nor can it be configured with any.

Today, every ~6h users are signed out of Sourcegraph and on Bitbucket see just:

This cannot be addressed on the customer's auth proxy side at all, so the only option for addressing this is via a bitbucket integration global option which displays all connection Error as Sign in to Sourcegraph.

For additional context, see https://sourcegraph.slack.com/archives/CMT39K56Z/p1589477130441100