cmd/cue: improve error messages for get go

[cueckoo]

Originally opened by @myitcv in cuelang/cue#701

As a follow up to #621 (and in preparation for #646) we should ensure we have good error messages for cue get go:

• in case a dependency is not available/cannot be resolved (i.e. properly surfacing go/packages errors)
• in case a target package cannot be properly type checked. Including:
  • syntax errors
  • type checking errors

[myitcv]

Dropping the now-defunct v0.4.x milestone from this issue, but leaving the zGarden label such that we come round to considering what milestone this should sit in.