Integration Procedure Try-Catch Does Not Surface Custom Apex .addError() Messages from DataMapper Steps to the Calling OmniScript

When a DataMapper Load step fails because an Apex trigger calls .addError() with a custom message, the following behavior is observed:
                                                                                                                                                                     
  1. The DML is rolled back as expected.
  2. The Try-Catch block in the Integration Procedure catches the failure.
  3. However, the Failure Response only contains a generic error such as:
  "UpsertError": "QuoteTrigger: execution of BeforeInsert"
  4. Any merge expression referencing the DataMapper's error output (e.g., %DataMapperStepName:UpsertError%) returns empty or null — the custom .addError() message is not present.
  5. The OmniScript calling the IP displays only the generic error, not the custom validation message from the trigger.

Note: When the IP is previewed or debugged in isolation (outside OmniScript), the custom error message may appear in the debug output. This discrepancy can cause confusion — the message is captured internally within the DataMapper's execution result but is not propagated into the Failure Response scope.