improves date_time_type error message for custom formats
improves date_time_type error message for custom formats

mlyundin/schematics,kstrauser/schematics,eatfirst/schematics,NerdWallet/schematics,promptworks/schematics,ee08b397/schematics,kaiix/schematics,not-napoleon/schematics,not-napoleon/schematics,kstrauser/schematics,promptworks/schematics,openprocurement/schematics,ee08b397/schematics