#369 Message box with no text if destination directory missing

[Terry O'Neill]

If the destination directory does not exist (for example, if the user sets the destination directory in the settings but then removes that directory in the file system) then Xena comes up with a blank error dialog when trying to normalise any file. The error itself in the console is a null pointer exception. In this situation Xena should tell the user that the destination directory does not exist instead.