How does one suppress or customize error messages in Forms?

One can either set the message level using the system variable SYSTEM.MESSAGE_LEVEL or trap errors using the ON-ERROR or ON-MESSAGE triggers.

MESSAGE_LEVEL: Set to 0, 5, 10, 15, 20, 25 to suppress all messages with severity below this level. The default level is 0. Messages with a level higher than 25 cannot be suppressed. See the "Forms Error Messages Manual" for more details about the various MESSAGE_LEVEL's:

0	Default value. All types of messages from the other levels of severity.
5	Reaffirms an obvious condition.
10	Indicates that the operator has made a procedural mistake.
15	Declares that the operator is attempting to perform a function for which the form is not designed.
20	Indicates a condition where the operator cannot continue an intended action due to a problem with a trigger or another outstanding condition.
25	Indicates a condition that could result in the form performing incorrectly.
>25	Indicates a message severity level that you cannot suppress via the SYSTEM.MESSAGE_LEVEL system variable.