Re: warn if GUC set to an invalid shared library

I would at least consider having the UX go something like:

postgres=# ALTER SYSTEM SET shared_preload_libraries = not_such_library;

ERROR: <paraphrase: your system will not reboot in its current state as that library is not present>.

HINT: to bypass the error please add FORCE before SET

postgres=# ALTER SYSTEM FORCE SET shared_preload_libraries = no_such_library;

NOTICE: Error suppressed while setting shared_preload_libraries.

That is, have the user express their desire to leave the system in a precarious state explicitly before actually doing so.