child_process: Emit err rather than throw for IPC write failure

lib/child_process.js

@@ -120,7 +120,8 @@ function setupChannel(target, channel) {
     var writeReq = channel.write(buffer, 0, buffer.length, sendHandle);
 
     if (!writeReq) {
-      throw errnoException(errno, 'write', 'cannot write to IPC channel.');
+      var er = errnoException(errno, 'write', 'cannot write to IPC channel.');
+      this.emit('error', er);
     }
 
     writeReq.oncomplete = nop;