handler.

This is called when an exception occurs and no exception
handler is set, and can be called by a custom exception
handler that wants to defer to the default behavior.

This default handler logs the error message and other
context-dependent information.  In debug mode, a truncated
stack trace is also appended showing where the given object
(e.g. a handle or future or task) was created, if any.

The context parameter has the same meaning as in
`call_exception_handler()`.
r