Return from get if getGetSocket errors

Currently, if an error is returned by `getGetSocket`, then `get` will still try to go ahead with the download. This will produce exceptions such as:

```
Error: 550 Failed to open file.

/Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:429
      socket.on('readable', function() {
             ^
TypeError: Cannot call method 'on' of undefined
    at /Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:429:14
    at f (/Users/sean.dwyer/ws/node_modules/jsftp/node_modules/once/once.js:16:25)
    at Array.cmdCallback [as 1] (/Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:461:23)
    at Ftp.parse (/Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:249:13)
    at Ftp.parseResponse (/Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:161:8)
    at Stream.<anonymous> (/Users/sean.dwyer/ws/node_modules/jsftp/lib/jsftp.js:132:24)
    at Stream.EventEmitter.emit (events.js:95:17)
    at ResponseParser.reemit (/Users/sean.dwyer/ws/node_modules/jsftp/node_modules/event-stream/node_modules/duplexer/index.js:70:25)
    at ResponseParser.EventEmitter.emit (events.js:95:17)
    at ResponseParser.<anonymous> (_stream_readable.js:746:14)
```