Allow exception types to be registered for format functions
These exceptions will be caught and message will be used for the ValidationError
Change errors during format checking to be attached to ValidationError in 'cause' attribute
Cleanups for FormatChecker error catching
Convert built in format checkers to provide original exception
Fix cls_checks args in docs
Update docs for format checkers
Oops
Merge pull request #76 from gazpachoking/format_errors

Allow exception types to be registered for format checker functions