[dashboard] Remaining issues in new start screen

[jankeromnes]

• Re-implement "repository may be private" errors (currently just "not found")
• Re-implement "out of hours" and "too many running workspaces" errors
• Wrong error message being displayed when running prebuild twice on same commit Wrong error message being displayed when running prebuild twice on same commit #3687
• Center error message and button on stopping workspace page Center error message and button on stopping workspace page #3654
• Fix double "Back to Dashboard" button in some cases(?)
• Implement stopped workspace pending changes summary (dropdown / alert)
• Actually handle/show ideFrontEndFailureCause error (can happen in rare cases, sent by supervisor via iframe event, but not sure how to trigger it)
• Improve error message when someone tries to open a merged PR with deleted branch Improve error message when someone tries to open a merged PR with deleted branch #3659
• Missing error log "Error: build failed: cannot build base image: Error response from daemon: Dockerfile parse error line 4: unknown instruction: ERROR" when trying to build https://github.com/gitpod-io/ruby-on-rails/tree/test-branch