Add optional source parameter to Arg.
Update docs, CHANGELOG, and bump dev version
Merge branch 'jmcarp-dev' into dev
Store error message string and exception on error in Flask.handle_error
Document flask error-handling, CHANGELOG, and bump dev version
Validator must return False for error to be raised

If None is returned, validation will pass
Allow extra metadata to be stored on an Arg
Bump version and update CHANGELOG