[UX] `sky check` should include fine grained error messages

[romilbhardwaj]

From a user:

.. when I ran sky check it said gcp was disabled--even though I have gcloud installed locally and connected to Google Cloud. I ran a few of the commands, but when I got to gcloud init I didn't want to wipe my current settings.

After looking at the docs, what it really wanted me to do was run gcloud auth application-default login. It would be nice to have more granular feedback about the checks that are run and what remains to be done, rather than installation instructions--especially incomplete ones.

[concretevitamin]

This is excellent feedback! Important for self-setup.