Custom Error instead of Appian default Pink Box Error

Well, there's already the common option (as of the last several years) wherein any error message a user sees will have all technical details replaced with a UUID they're told to report to their administrators, which you can then go and match with the matching error in the error logs.  Do you have that enabled?

Otherwise, the only way to prevent pink box errors is... remove the error conditions that cause them.  And (other than the above setting) there's no setting or override to show something "custom".  I know it can be hard to weed out 100% in a sufficiently large system, but in most cases you should be able to keep them down to very few (and fix the ones that anyone encounters).

Replacing the pink box error with custom one is a use case we can't simply or directly implement. Workarounds involve design approaches that reduces the occurrence of such errors is the best and closest implementation we can do to handle these. The better the data and rule design the lower are chances for those pink box errors. 

No global way to replace the pink box, and SAIL has no try/catch.