test-file-saver: print error when operation not supported
Remove the GtkSourceFileLoader:location property
FileSaver: location -> target-location

gtk_source_file_saver_get_location() returns the location where the
contents is saved. It is either the target-location, or the
GtkSourceFile's location. In GeditTab this function is used at several
places, and a get_target_location() would not be convenient.