thought it should, but forgot to fix Report to use
ReferenceProperty in the way I thought it should! Oops.
Added some tests to ensure Report is working property with
custom ID types in object_refs.
partly inspired by PR #263. This resulted in some error message
format changes (an improvement, I think), which caused some
unit test breakage. Removed those asserts from the unit tests,
since tests shouldn't be testing human-targeted error messages.