Added error callback to download prototype

Fixing this issue: #18

If there is an error in the download function, it would break when trying to use "sftp". This fixes that.

Author: KSoto

lib/client.js

@@ -279,6 +279,10 @@ Client.prototype.download = function(src, dest, callback) {
   var self = this;
 
   self.sftp(function(err,sftp){
+    if (err) {
+      return callback(err);
+    }
+
     var sftp_readStream = sftp.createReadStream(src);
     sftp_readStream.on('error', function(err){
       callback(err);