more clear error message when file not exist

when the file is not exist.
the error msg is
```
events.js:87
      throw Error('Uncaught, unspecified "error" event.');
            ^
Error: Uncaught, unspecified "error" event.
    at Error (native)
    at Transform.EventEmitter.emit (events.js:87:13)
```
it's hard to read.

after this change, the message is
```
events.js:85
      throw er; // Unhandled 'error' event
            ^
Error: ENOENT, no such file or directory '...'

```