core: Remove IllegalOperationException

Since IllegalOperationException is never caught directly, it can be
replaced with the standard Java IllegalArgumentException which carries
the exact same meaning and also extends RuntimeException.

Change-Id: Ib0505a889a60ea86e0473ed8651c1e47eee148c9
Signed-off-by: Allon Mureinik <amureini@redhat.com>