I would like to replace standard ORA-xxxxx messages with custom ones in Oracle Application Express. For example instead of this:
to have something like "You have left some fields blank, try again". What is a best way to do this?
In Apex specifically, use validations. You can specify them per field or use a custom one that checks all the fields at once.
http://docs.oracle.com/cd/E23903_01/doc/doc.41/e21674/bldr_validate.htm#BABCDBDE