cmd/go: 'go list -mod=readonly -e' should attribute errors to individual missing packages

[bcmills]

This issue is pulled out from #29452 (comment).

I believe that go list -mod=readonly -e currently fails the entire command (and does not generate output for any package) if any package in the import graph cannot be resolved from a module already present in the go.mod file.

It should not: instead, it should report errors for the specific packages that could not be resolved from the existing dependencies.

CC @jayconrod

[gopherbot]

Change https://golang.org/cl/209037 mentions this issue: cmd/go: test that 'go list -e -mod=readonly' reports errors correctly