gs-plugin-loader: Tidy up anything_ran handling

anything_ran is set to indicate that at least one plugin handled a certain job (even if it returned an error). However, it’s not set on all error paths in gs_plugin_loader_call_vfunc(). Ensure it is, to avoid an incorrect error potentially being reported.

Signed-off-by: Philip Withnall withnall@endlessm.com