Display improper zip structure message to non-console users

If a zip file is improperly structured, a message will be displayed when
the player loads the level list.

This will only display the last-displayed improper zip, because there
only needs to be one displayed at a time. Also because doing anything
more would most likely require heap allocation, and I don't want to do
that.

desktop_version/src/editor.cpp

@@ -229,6 +229,8 @@ void editorclass::getDirectoryData(void)
 
     ListOfMetaData.clear();
 
+    FILESYSTEM_clearLevelDirError();
+
     loadZips();
 
     FILESYSTEM_enumerateLevelDirFileNames(levelMetaDataCallback);