errors: make dns errors consistent

Right now the hostname could in some cases be missed, depending on
the libuv error number. This makes sure there the hostname is always
added, if available.

PR-URL: #19754
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/dns.js

@@ -229,7 +229,7 @@ function resolver(bindingName) {
     req.oncomplete = onresolve;
     req.ttl = !!(options && options.ttl);
     var err = this._handle[bindingName](req, name);
-    if (err) throw dnsException(err, bindingName);
+    if (err) throw dnsException(err, bindingName, name);
     return req;
   }
   Object.defineProperty(query, 'name', { value: bindingName });

lib/internal/errors.js

@@ -556,20 +556,20 @@ function exceptionWithHostPort(err, syscall, address, port, additional) {
  * @returns {Error}
  */
 function dnsException(code, syscall, hostname) {
-  let message;
-  // FIXME(bnoordhuis) Remove this backwards compatibility nonsense and pass
-  // the true error to the user. ENOTFOUND is not even a proper POSIX error!
-  if (code === UV_EAI_MEMORY ||
-      code === UV_EAI_NODATA ||
-      code === UV_EAI_NONAME) {
-    code = 'ENOTFOUND'; // Fabricated error name.
-  }
-  if (typeof code === 'string') { // c-ares error code.
-    message = `${syscall} ${code}${hostname ? ` ${hostname}` : ''}`;
-  } else {  // libuv error number
-    code = lazyInternalUtil().getSystemErrorName(code);
-    message = `${syscall} ${code}`;
+  // If `code` is of type number, it is a libuv error number, else it is a
+  // c-ares error code.
+  if (typeof code === 'number') {
+    // FIXME(bnoordhuis) Remove this backwards compatibility nonsense and pass
+    // the true error to the user. ENOTFOUND is not even a proper POSIX error!
+    if (code === UV_EAI_MEMORY ||
+        code === UV_EAI_NODATA ||
+        code === UV_EAI_NONAME) {
+      code = 'ENOTFOUND'; // Fabricated error name.
+    } else {
+      code = lazyInternalUtil().getSystemErrorName(code);
+    }
   }
+  const message = `${syscall} ${code}${hostname ? ` ${hostname}` : ''}`;
   // eslint-disable-next-line no-restricted-syntax
   const ex = new Error(message);
   // TODO(joyeecheung): errno is supposed to be a number / err, like in