flutter dashboard should show error when failed trying to unblock the tree

### Steps to reproduce

1. visit flutter dashboard: https://flutter-dashboard.appspot.com/#/build
2. Login with your github
3. unlink Google account (from the profile photo on top right corner)
4. click a icon column header of a test
5. click "unblock tree" with a github issue link

### Expected results

Website should fail but also tell you why it fails (e.g. "Link Google Account is required")

### Actual results

Website logs you out

### Code sample

NA

### Screenshots or Video

NA

### Logs

NA

### Flutter Doctor output

NA

Start at the Flutter dashboard build page and reproduce the profile-account unlink and “unblock tree” flow described in the issue. Trace the dashboard entry point for the unblock-tree action and verify that a failed request shows the required-account explanation instead of logging the user out; done means the failure is visible and actionable.

Written by the indexing model from the issue text.