Add exceptions for more detailed error handling

[kbasten]

[harishanchu]

Thank you @kbasten, I will look into it this weekend.

[harishanchu]

Hi @kbasten,

It makes sense to throw named exceptions from connection setup methods as it was throwing Exceptions even before.

Regular ftp methods are coded in way that it will always catch the exception and return false if action fails. This is convenient that we don't have to worry about wrapping regular ftp methods with try catch where ever they are called. Convince me why should we throw exceptions in regular ftp methods or please revert the changes done to changeDir method so that I can accept and merge this pull request.