I've used this error handler as the most simple to get done what has to be done.

on error do ezerror with message()


PROCEDURE Ezerror
PARAMETERS cMessage
lnVal = messagebox("AN ERROR OCCURED" + CHR(13) + CHR(13) + cMessage + CHR(13) + CHR(13),2)
DO CASE
CASE lnval = 3
CANCEL
CASE lnval=4
RETRY
CASE lnval=5
RETURN

ENDCASE
RETURN

No constants, no subroutines, no external routines. Simple as it gets.