check that error conditions produce human-readable error messages

[grahamgower]

This might be tough to test, but we should be able to check some things systematically. Like if there's a missing "end_time" in an epoch, we should check that the deme ID and epoch index are in the error message. We might also consider having a collection of erroneous input files (as already suggested for the spec repository: popsim-consortium/demes-spec#27), and creating a table of error messages for each input file, so we can manually inspect the error messages for any obvious improvements or regressions.