Make WikibaseClient and WikibaseLib messages more user-friendly

We split this from I0b4324f600b26aa9b0f3e433878f6f013a3fb11d.
This contains the undisputed changes and should be merged soon.
What's left in the depending patch needs more discussion.

Bug: 62778
Change-Id: I3f3704abbddebc439d118c0004df0801179a542e