errors: provide defaults for unmapped uv errors

lib/internal/errors.js

@@ -327,6 +327,8 @@ function lazyUv() {
   return uvBinding;
 }
 
+const uvUnmappedError = ['UNKNOWN', 'unknown error'];
+
 function uvErrmapGet(name) {
   uvBinding = lazyUv();
   if (!uvBinding.errmap) {
@@ -346,7 +348,7 @@ function uvErrmapGet(name) {
  * @returns {Error}
  */
 function uvException(ctx) {
-  const [ code, uvmsg ] = uvErrmapGet(ctx.errno);
+  const [ code, uvmsg ] = uvErrmapGet(ctx.errno) || uvUnmappedError;
   let message = `${code}: ${ctx.message || uvmsg}, ${ctx.syscall}`;
 
   let path;
@@ -404,7 +406,7 @@ function uvException(ctx) {
  * @returns {Error}
  */
 function uvExceptionWithHostPort(err, syscall, address, port) {
-  const [ code, uvmsg ] = uvErrmapGet(err);
+  const [ code, uvmsg ] = uvErrmapGet(err) || uvUnmappedError;
   const message = `${syscall} ${code}: ${uvmsg}`;
   let details = '';
 

test/parallel/test-uv-unmapped-exception.js

@@ -0,0 +1,26 @@
+// Flags: --expose-internals
+'use strict';
+require('../common');
+const assert = require('assert');
+const { uvException, uvExceptionWithHostPort } = require('internal/errors');
+
+{
+  const exception = uvException({ errno: 100, syscall: 'open' });
+
+  assert.strictEqual(exception.message, 'UNKNOWN: unknown error, open');
+  assert.strictEqual(exception.errno, 100);
+  assert.strictEqual(exception.syscall, 'open');
+  assert.strictEqual(exception.code, 'UNKNOWN');
+}
+
+{
+  const exception = uvExceptionWithHostPort(100, 'listen', '127.0.0.1', 80);
+
+  assert.strictEqual(exception.message,
+                     'listen UNKNOWN: unknown error 127.0.0.1:80');
+  assert.strictEqual(exception.code, 'UNKNOWN');
+  assert.strictEqual(exception.errno, 100);
+  assert.strictEqual(exception.syscall, 'listen');
+  assert.strictEqual(exception.address, '127.0.0.1');
+  assert.strictEqual(exception.port, 80);
+}