Change ContactStore dependents to handle of ContactError execptions
SNAEventHandler.find_contact() does not need ContactStore to be guarded for ContactError, moved out of try-catch block
GoApplicationMixin.get_contact_for_message() should accept an optional create arg, allowing the contact to be created if it does not yet exist
Create a contact in SurveyApplication.consume_user_message() and .end_session() when retrieving the contact from a message if the contact does not yet exist
Merge branch 'feature/issue-218-contactsstore-dependents-should-handle-contacterror-exceptions-properly' into develop