Error handling?

[dgutov]

If any of the options are wrong (hostname, username or password), EM::FtpClient::Session doesn't call the callback, and EM::FtpClient::SyncSession similarly freezes.

I tried to fix that by simply adding errback to @control_connection in Session#initialize, but:

1. It doesn't get called anyway if the host is wrong.
2. If it's just the username that is wrong, the callback gets called, but I have no way to distinguish this situation from when everything went right. E.g., control_connection.error? still returns false.

[naksu]

Here's a fix for SyncSession: naksu@2a8a43d
Pull request #5