domain: Do not use uncaughtException handler

This adds a process._fatalException method which is called into from
C++ in order to either emit the 'uncaughtException' method, or emit
'error' on the active domain.

The 'uncaughtException' event is an implementation detail that it would
be nice to deprecate one day, so exposing it as part of the domain
machinery is not ideal.

Fix #4375
domain: use camelCase instead of snake_case

While it's true that error objects have a history of getting snake_case
properties attached by the host system, it's a point of confusion to
Node users that comes up a lot.  It's still 'experimental', so best to
change this sooner rather than later.
test-message: Add setTimeout and nextTick message tests
domain: Add helpful comment to process._fatalException