cmd/cue: module registry errors should show which registry was being used

[mvdan]

A user on Slack just asked why they got this error when they configured a Docker credential helper for their ECR registry:

cannot put module: cannot make scratch config: 401 Unauthorized: unauthorized: bearer auth required

"bearer auth required" is an error message from the central registry at registry.cue.works, the default $CUE_REGISTRY. So the user wasn't setting $CUE_REGISTRY properly when calling cue mod publish.

The user was making a mistake here, but our error did not help either; the mistake would have been much more apparent if the error instead mentioned the registry being published to, like:

cannot put module to registry "registry.cue.works": cannot make scratch config: 401 Unauthorized: unauthorized: bearer auth required

Or, better yet, mention both the module and registry:

cannot put module "foo.com/bar" to registry "registry.cue.works": cannot make scratch config: 401 Unauthorized: unauthorized: bearer auth required

The error message at that point is getting a bit lengthy, so I suggest that we omit "cannot make scratch config", given that it's almost entirely pointless to end users:

cannot put module "foo.com/bar" to registry "registry.cue.works": 401 Unauthorized: unauthorized: bearer auth required

[myitcv]

As a thought, if we implement cue env could we keep the error message brief? Bug reports would then be directed to include the output of cue env, much like Go does.