[UX] Print useful message when image id not found

[cblmemo]

Fixes #2519 by adding some messages to help the user debug. Most of the messages are pasted from our yaml-spec.rst.

Current docker implementation won't check whether the docker image exists, so no message for docker is added in this PR.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

(sky-dev) ➜  skypilot git:(ux-image-id) $ sky launch --cloud gcp --image-id some-random-string
ValueError: Image 'some-random-string' not found in GCP.

To find GCP images: https://cloud.google.com/compute/docs/images
Format: projects/<project-id>/global/images/<image-name>
Example: projects/deeplearning-platform-release/global/images/common-cpu-v20230615-debian-11-py310

To find machine images: https://cloud.google.com/compute/docs/machine-images
Format: projects/<project-id>/global/machineImages/<machine-image-name>

You can query image id using: gcloud compute images list --project <project-id> --no-standard-images
To query common AI images: gcloud compute images list --project deeplearning-platform-release | less

(sky-dev) ➜  skypilot git:(ux-image-id) $ sky launch --cloud aws --image-id some-random-string --region us-east-1
ValueError: Image 'some-random-string' not found in AWS region us-east-1.

To find AWS AMI IDs: https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-images.html#examples
Example: ami-0729d913a335efca7

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

Thanks @cblmemo. Minor UX nits. I guess it's fine to leave docker:xxx to the future?

[concretevitamin]

Thanks @cblmemo, LGTM.

[concretevitamin]

Short comment? Add a blank line to separate it from L88

[cblmemo]

Sorry wdym by 'short comment'? Did you mean to short the following string so we could get rid of the # pylint: disable=line-too-long?

[concretevitamin]

# Describe this var briefly.
USER_PORTS_FIREWALL_RULE_NAME = 'sky-ports-{}'

:)