Improve error messages about invalid/missing pkg.opam files
Improve error messages about invalid/missing pkg.opam files

janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,rdavison/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,rdavison/jbuilder