If libunwind is unavailable, make libunwind_virtual_unwind return an
error. Likewise if there was no PE exception info and we didn't try
unwinding using libunwind, we need to return an error when realizing
we were unable to unwind, instead of just warning and returning
success.

This fixes hangs/infinite loops on crashes when unwinding fails.
Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: André Hentschel <nerv@dawncrow.de>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>