Make WikibaseRepo messages more user-friendly

We split this from Id3942ea6ca2fe9e296ac8de3103834328c57e468.
This contains the undisputed changes and should be merged soon.
What's left in the depending patch needs more discussion.

Bug: 62778
Change-Id: I300b145c21d277f892a5864ae70daefaf0787cb9
Merge "Make WikibaseRepo messages more user-friendly"