MID(
 {!$Flow.FaultMessage},
 FIND('FIELD_CUSTOM_VALIDATION_EXCEPTION: ', {!$Flow.FaultMessage} )+34,
 (
   LEN({!$Flow.FaultMessage} ) -
   (FIND('FIELD_CUSTOM_VALIDATION_EXCEPTION: ', {!$Flow.FaultMessage} )+34)
 ) -
 (
   LEN({!$Flow.FaultMessage} ) -
   FIND('. For details, see API Exceptions', {!$Flow.FaultMessage} )
 )
)

FIND('. You can look up ExceptionCode', {!$Flow.FaultMessage} )