core: Require TLS cert to be loaded if --require-ssl is used

core: Require TLS cert to be loaded if --require-ssl is used
If the user specifies --require-ssl, but the core cannot load a
SSL/TLS certificate for any reason, the core will throw an
exception and quit. This fixes a minor security vulnerability
where previously, the core would simply fall back to plaintext
mode and not offer encrypted connections at all.