webgui: fix error in canvas painter commands cleanup
webgui: fix error in canvas painter commands cleanup

Add error output when canvas fails to produce image
root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root,karies/root,karies/root,olifre/root,karies/root,olifre/root