cmd/go: go mod tidy, vendor hide module load errors

[rsc]

The module loader leaves some problems for the standard loader to find, like not being able to resolve imports. But 'go mod tidy' and 'go mod vendor' do not run the standard loader, so those problems go undiagnosed. Maybe we should look for them explicitly.

For example make a file x.go that says import "nonexist". go mod tidy succeeds quietly.

[kevinburke]

#27868 is possibly a duplicate of this issue; I'm not positive.

[jsternberg]

We have run into this issue where using go mod tidy on a machine that doesn't have bzr, but that has bzr dependencies, will just not mention that it didn't work correctly.

We caught it because our CI system is now running go mod tidy and it has bzr installed.