[WFLY-2584] Throw an IllegalArgumentException is a user attempts to switch to a role that does not exist.

Note: This is only thrown for users that would have the ability to switch roles if the role actually existed - other users it still fails silently to stop them from using this to discover which roles actually exist.
Merge pull request #5821 from darranl/WFLY-2584

[WFLY-2584] Throw an IllegalArgumentException is a user attempts to swit...