server: specific err for config w/o cert resolver

When we made the server config builder use an out param for the built
config it allowed us to return an error when building a config without
a cert resolver. At the time we used `RUSTLS_RESULT_GENERAL`, but this
offers no significant hint at the root cause. This commit introduces
a new `RUSTLS_RESULT_NO_CERT_RESOLVER` error and updates the impl and
unit test to use it.

src/error.rs

@@ -63,6 +63,7 @@ u32_enum_builder! {
         NoServerCertVerifier => 7015,
         NoDefaultCryptoProvider => 7016,
         GetRandomFailed => 7017,
+        NoCertResolver => 7018,
 
         // From https://docs.rs/rustls/latest/rustls/enum.Error.html
         NoCertificatesPresented => 7101,
@@ -499,6 +500,9 @@ impl Display for rustls_result {
             GetRandomFailed => {
                 write!(f, "failed to get random bytes from the crypto provider")
             }
+            NoCertResolver => {
+                write!(f, "no certificate resolver was configured")
+            }
 
             CertEncodingBad => Error::InvalidCertificate(CertificateError::BadEncoding).fmt(f),
             CertExpired => Error::InvalidCertificate(CertificateError::Expired).fmt(f),

src/rustls.h

@@ -26,6 +26,7 @@ enum rustls_result {
   RUSTLS_RESULT_NO_SERVER_CERT_VERIFIER = 7015,
   RUSTLS_RESULT_NO_DEFAULT_CRYPTO_PROVIDER = 7016,
   RUSTLS_RESULT_GET_RANDOM_FAILED = 7017,
+  RUSTLS_RESULT_NO_CERT_RESOLVER = 7018,
   RUSTLS_RESULT_NO_CERTIFICATES_PRESENTED = 7101,
   RUSTLS_RESULT_DECRYPT_ERROR = 7102,
   RUSTLS_RESULT_FAILED_TO_GET_CURRENT_TIME = 7103,

src/server.rs

@@ -356,7 +356,7 @@ impl rustls_server_config_builder {
             let mut config = if let Some(r) = builder.cert_resolver {
                 base.with_cert_resolver(r)
             } else {
-                return rustls_result::General;
+                return rustls_result::NoCertResolver;
             };
             if let Some(ss) = builder.session_storage {
                 config.session_storage = ss;
@@ -816,7 +816,7 @@ mod tests {
         let mut config = null();
         let result =
             rustls_server_config_builder::rustls_server_config_builder_build(builder, &mut config);
-        assert_eq!(result, rustls_result::General);
+        assert_eq!(result, rustls_result::NoCertResolver);
         assert!(config.is_null());
     }