campaigns: Cleanup some little things

Created by: eseliger

I found these while browsing the code, trying to understand the selection of auth tokens for the syncer.

• The repoStore.Get method already returns an error with that error message, so no need to wrap it another time. Saves us a copy-pasted helper
• Handle potential nil index access in buildChangesetSource
• Correctly store error in syncChangeset so transactions are correctly rolled back