Review log message returned when creating an unknown exception from ValidationError

[iNinja]

When implementing a delegate in the new validation model using ValidationParameters, users can return the type of the exception that should be returned when calling GetException() on the ValidationError object.

The library implements the creation of the known and used exception types, but there is a scenario where the user can pass an unknown exception type. In this situation, the library returns a SecurityTokenException instead of the specified type, providing the following message along with the unknown exception type and the original message that was attached to it:

IDX10002: Unknown exception type returned. Type: '{0}'. Message: '{1}'.

We need to review this message to ensure it conveys the right information to users, and possibly create an aka.ms page with more information explaining how they can implement their own ValidationError subclass and override GetException() to return their custom exception.