It can happen that the download-directory for the file-transfer is not valid.
If a user try to receive a file with a invalid directory, he doesn't receive a useful error message.
Give error-message if download directory is invalid and user try to receive a file.
To improve the usability on this error, the error-message provide a direct link to the file-transfer preferences.
This link need the ability of the preference-window, to pre-select a preference.
Fixed wrong return type that was meant for debug issues.