We can see BpmErrors on bg-ca.log when we define customer script as follows and run the process.
Custom Script Properties:
<Please see attached file for image>

Error:
ERROR YYYY-MM-DD HH:MM:SS,SSS [Custom script execution pool-8-thread-1] utilities.BpmErrors
(clarity:test2:5169137__284A85DB-8AED-4D66-97E3-913B87E5A226:none)
Begin Error Information:
Step Action Id: 5014003
Process Instance Id: 5001001
Step Instance Id: 5001003
Error: bpm.customScriptDone
Error Code: bpm.customScriptDone
End Error Information