Handle error, pass to callback

This allows the client of scp2 module to handle error accordingly.

Otherwise it will break:

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: connect ETIMEDOUT
    at errnoException (net.js:901:11)
    at Object.afterConnect [as oncomplete] (net.js:892:19)
Merge pull request #6 from cliffano/patch-1

Handle error, pass to callback