Remove error parameter of the "loaded" and "saved" GeditDocument signals

The error parameter has been removed since the port of gedit to
GtkSourceFile.
Don't use gedit_document_set_location() (deprecated)

Use gtk_source_file_set_location() instead.
Don't use gedit_document_get_location() (deprecated)

Use gtk_source_file_get_location() instead.
GtkSourceView >= 3.13.2 is required

For GtkSourceFile.