GitHub code host doesn't report problems when adding invalid tokens

Created by: artemruts

Problem

When adding invalid tokens (think bla-bla-42) for GitHub code host, backend reports code host as successfully added. GraphQL mutation is AddExternalService.

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Based on the architecture, I'd guess this happens due to indirection and throttling @ github-proxy and the errors will be eventually reported when code hosts starts to sync.

Steps to reproduce

0. Make sure your user has allowUserExternalServicePublic tag set
1. Navigate to code host connections page https://sourcegraph.com/users/<your user>/settings/code-hosts
2. Open Browser DevTools and monitor network
3. Add bad-token personal access token to GitHub code host connection
4. Examine 200 response from https://sourcegraph.com/.api/graphql?AddExternalService, it will have warning and lastSyncError set as null
5. ⚠️ Instead we should have warning or lastSyncError have an error message.

Screenshots