child_process: add errno property to exceptions

In case of a write failure when using fork() an error would be thrown. The
thrown exception was missing the `errno` property.

lib/child_process.js

@@ -132,7 +132,7 @@ function setupChannel(target, channel) {
     var writeReq = channel.write(buffer, 0, buffer.length, sendHandle);
 
     if (!writeReq) {
-      throw new Error(errno + 'cannot write to IPC channel.');
+      throw errnoException(errno, 'write', 'cannot write to IPC channel.');
     }
 
     writeReq.oncomplete = nop;
@@ -471,11 +471,15 @@ ChildProcess.prototype.spawn = function(options) {
 };
 
 
-function errnoException(errorno, syscall) {
+function errnoException(errorno, syscall, errmsg) {
   // TODO make this more compatible with ErrnoException from src/node.cc
   // Once all of Node is using this function the ErrnoException from
   // src/node.cc should be removed.
-  var e = new Error(syscall + ' ' + errorno);
+  var message = syscall + ' ' + errorno;
+  if (errmsg) {
+    message += ' - ' + errmsg;
+  }
+  var e = new Error(message);
   e.errno = e.code = errorno;
   e.syscall = syscall;
   return e;