Interface for model error handlers. A model error is an exception that is
passed up the Ptolemy II hierarchy for handling until a container with
a registered error handler is found. If there is no registered error
handler, then the error is ignored. It is like throwing an exception, except
that instead of unraveling the calling stack, it travels up the Ptolemy II
hierarchy.
A typical use of this facility is where an actor does the following:
handleModelError(this, new IllegalActionException(this, message));
instead of this:
throw new IllegalActionException(this, message);
The former allows a container in the hierarchy to intercept the
exception, whereas the latter simply throws the exception.