Replace exceptions thrown by signal and wait methods with [[nodiscard]] return
values.
This is mostly a refactoring, but there is a slight change of behavior if
AbortShutdown function fails. The original behavior which was unintentionally
changed in #27861 is restored, so it now triggers an assert failure again
instead of throwing an exception. (The AbortShutdown function is only ever
called in the the GUI version of Bitcoin Core when corruption is detected on
loading and the user tries to reindex.)
Problems with using exceptions were pointed out by MarcoFalke in
https://github.com/bitcoin/bitcoin/pull/27861#discussion_r1255496707.
This change helps generalize shutdown code so an interrupt can be
provided to libbitcoinkernel callers. This may also be useful to
eventually de-globalize all of the shutdown code.
Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>
