Reorder the entry for die, moving discussion of the exit code later.

Reorder the entry for die, moving discussion of the exit code later.

Change to start with "C<die> raises an exception." and phrase in terms of
handling exceptions, and then how uncaught exceptions result in process exit.
Do not give details of the exit code in the first paragraph. Move this to a
later paragraph, clarify that $! is often unpredictable, and stress that 255
is the last resort exit code.