This commit improves the discoverability of exceptions that are "fatal"
to Reactor by:

• adding Exceptions.isFatal and Exceptions.isJvmFatal methods
• adding logging to Exceptions.throwIfFatal and
  Exceptions.throwIfJvmFatal just before a fatal exception is thrown

This should at least help pinpointing such occurrences from the logs,
in cases where the thrown exception bubble all the way up the stack of
a thread with no uncaughtExceptionHandler for example.

Fixes #3111.