Fix confusing error message in multi-select RegistryFields

Adding an unknown string to .names previously triggered a
confusing error, because the code intended to handle that error
invoked SelectionSet.__getitem__ (which doesn't exist) instead of
ConfigChoiceDict.__getitem__.
Minor cleanups for configChoiceField.py (unused variables, whitespace)
Improve error message for unknown key in RegistryField
Merge branch 'tickets/DM-1505'