Surface meaningful error message when repo-updater is being deployed

Created by: jplahn

When repo-updater is being deployed and a user attempts to make changes to their repositories, they're presented with a technical error message without any further context, and an action that doesn't resolve the problem.

We're about to ask a lot of folks to go through this flow, and without zero-downtime deployments for repo-updater, it's likely that new users will run into this problem. With an opaque error message like this, it suggests that something went wrong and we don't know what—which isn't great for perceived trust with users' private code.

Proposal If we can identify that this is caused by repo-updater deployments, we should present an error message on Cloud that follows our content guidelines.

Very tentatively, something along the lines of this would better explain what the problem is and what to expect, and hopefully help to build trust until we have zero-downtime deployments.

Could not load repositories A system deployment is likely underway. Please wait a few moments, then try reloading the page. The error was: error in SyncExternalService for service "GITHUB" with ID 1119: Post "http://repo-updater:3182/sync-external-service": dial tcp 10.165.5.97:3182: connect: connection refused.