Fix 'email' custom type example on docs: - Returning the same string instead of True draws a clearer picture of the expected behaviour of a custom type function. - Replace 'ValidationError' with a built-in exception.
Fix 'email' custom type example on docs:
- Returning the same string instead of True draws a clearer picture of the expected behaviour of a custom type function.
- Replace 'ValidationError' with a built-in exception.

ueg1990/flask-restful,sergeyromanov/flask-restful,samarthmshah/flask-restful,marrybird/flask-restful,ihiji/flask-restful,whitekid/flask-restful,santtu/flask-restful,frol/flask-restful,Khan/flask-restful,codephillip/flask-restful,FashtimeDotCom/flask-restful,justanr/flask-restful,expobrain/flask-restful,ankravch/flask-restful,flask-restful/flask-restful,mitchfriedman/flask-restful,elatomo/flask-restful,mackjoner/flask-restful,gonboy/flask-restful,red3/flask-restful,liangmingjie/flask-restful