crypto: improve errors in DiffieHellmanGroup

1. The DiffieHellmanGroup class is only instantiated from within
   Node.js, which always passes exactly one argument.
2. Use the existing ERR_CRYPTO_UNKNOWN_DH_GROUP error code for the
   existing "Unknown group" error. The message has not been changed
   to prevent breaking existing applications.

PR-URL: nodejs#31445
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: tniessen

src/node_crypto.cc

@@ -5158,18 +5158,15 @@ void DiffieHellman::DiffieHellmanGroup(
   Environment* env = Environment::GetCurrent(args);
   DiffieHellman* diffieHellman = new DiffieHellman(env, args.This());
 
-  if (args.Length() != 1) {
-    return THROW_ERR_MISSING_ARGS(env, "Group name argument is mandatory");
-  }
-
+  CHECK_EQ(args.Length(), 1);
   THROW_AND_RETURN_IF_NOT_STRING(env, args[0], "Group name");
 
   bool initialized = false;
 
   const node::Utf8Value group_name(env->isolate(), args[0]);
   const modp_group* group = FindDiffieHellmanGroup(*group_name);
   if (group == nullptr)
-    return env->ThrowError("Unknown group");
+    return THROW_ERR_CRYPTO_UNKNOWN_DH_GROUP(env, "Unknown group");
 
   initialized = diffieHellman->Init(group->prime,
                                     group->prime_size,

test/parallel/test-crypto-dh.js

@@ -389,7 +389,11 @@ assert.throws(
   function() {
     crypto.getDiffieHellman('unknown-group');
   },
-  /^Error: Unknown group$/,
+  {
+    name: 'Error',
+    code: 'ERR_CRYPTO_UNKNOWN_DH_GROUP',
+    message: 'Unknown group'
+  },
   'crypto.getDiffieHellman(\'unknown-group\') ' +
   'failed to throw the expected error.'
 );