SFTP plugin should give a nicer error when it won't work because of lacking permissions
If we don't have the required filesystem permissions on the Android side, the SFTP plugin won't be loaded. However, the capabilities will still return that SFTP is supported, so someone might try to use the plugin from the desktop client.
If this happens, it silently doesn't work (only a log is printed on Android saying that the packet was ignored). We should be able to detect that permissions are missing on the Android side and show an error with an explanation on the desktop side when we try to access the phone.
Related bugzilla bug: https://bugs.kde.org/show_bug.cgi?id=475795