fixed bug when an FTP Exception is thrown

A WebException can have several response types, its not necessarily always an HttpWebResponse
This looks like the only way of dealing with it really as Ftp/HttpWebResponse have the properties we want on them, but the base class doesn't. 

There is also a FileWebResponse class, but it doesn't have StatusCode or StatusDescription properties so we can't really do anything with it. We should probably change the else to else if (webError.Response is HttpWebResponse) just to be safe.