Reading lists: fix API error messages some more
Reading lists: fix API error messages some more

Messed this up in #956, the action API automatically removes
any 'apierror-' prefix from the error message when converting it
to an error code.

Bug: T182538

wikimedia/mediawiki-services-restbase,milimetric/restbase,d00rman/restbase,cscott/restbase,d00rman/restbase,Pchelolo/restbase,Pchelolo/restbase,wikimedia/restbase,wikimedia/mediawiki-services-restbase,cscott/restbase,wikimedia/restbase,milimetric/restbase