Improve handling for "no roots in server config"

[jsha]

Right now, if you call rustls_server_config_builder_build on a config builder where no cert resolver has been set, it will return NULL. This is a little inconsistent with other functions, where returning NULL is reserved for panics or null inputs.

Instead we should do one of two things:

• Make cert resolver a mandatory argument to rustls_server_config_builder_new and rustls_server_config_builder_new_custom, or
• Make rustls_server_config_builder_build() take an output parameter to write the built server config, and return a rustls_result, which may return a new error value RUSTLS_RESULT_NO_CERT_RESOLVER.

[cpu]

Make rustls_server_config_builder_build() take an output parameter to write the built server config, and return a rustls_result, which may return a new error value RUSTLS_RESULT_NO_CERT_RESOLVER.

The 0.14.0 release changed the server config builder to use an out-param, but we're currently returning a RUSTLS_RESULT_GENERAL error for the case described here:

rustls-ffi/src/server.rs

Lines 356 to 360 in eb3ccfa

	let mut config = if let Some(r) = builder.cert_resolver {
	base.with_cert_resolver(r)
	} else {
	return rustls_result::General;
	};

I will open a fix to use a better error result.