MutableException check requires class to explicitly extend some other class (#60)

My attempt to fix #60.

In order to be exception/error in Java, a class must inherit from Throwable. As in Checkstyle we don't have an access to types, at least we can check that a class extends anything.

What do you think? I'm not sure if we should introduce pattern for class being extended. E.g. java.lang.ThreadDeath is an error, but doesn't have Error in name.