Minor message refactoring, and tweaking of scripts

Tests are no longer sensitive to the version of the
nexson_validator being run, because the version
part of the JSON export is being removed with sed.
mandatory key warning in a class. Generic error fmethods
more messages to warning message classes.
MissingExpectedListWarning
DuplicatingSingletonKeyWarning and MissingExpectedListWarning
flagging severity code in emit_error and add more Warning classes
NoRootNodeWarning and TipWithoutOTUWarning
PropertyValueNotUsefulWarning added
ConflictingPropertyValuesWarning added
UnrecognizedPropertyValueWarning added
More warning message classes
UnrecognizedTagWarning added