Platform Exceptions have no message

While analysing FINERACT-1060, I thought it was a bit curious that that UserNotFoundException showed up as UserNotFoundException: null, instead of something like User with name 'system' does not exist.

Looking into this further, I suspect that this is related to that entire business which Fineract (apparently) has of of exception with "user" messages that can be translated (somehow, somewhere).

What's missing & confusing, at least if these exceptions are then also used for error logging, is that the "normal" exception message is never set.

It should be fairly simply to actually set it to be that "defaultUserMessage" (especially after doing some refactoring which I thought would be natural to do after Manthan Surkar work in https://github.com/apache/fineract/pull/1079 for FINERACT-942), so I'll propose a PR related to this.