UX: drop image_id warning, and print a hint for a corner case.

[concretevitamin]

User got this error

I 05-26 15:09:51 optimizer.py:1002] No resource satisfying AWS({'T4': 8}, image_id={'us-west-2': 'ami-1234'}) on AWS.
sky.exceptions.ResourcesUnavailableError: No launchable resource found for task train-1234. Region: us-west-2.
This means the catalog does not contain any instance types that satisfy this request.
To fix: relax or change the resource requirements.
Hint: 'sky show-gpus --all' to list available accelerators.
'sky check' to check the enabled clouds.

and mentioned

But the actual reason was because I did not configure ssh_proxy_command in us-west-2

This change adds a friendlier warning.

Repro:

# ~/.sky/config.yaml
aws:
  ssh_proxy_command:
    us-east-2: ...

# ~/test.yaml
resources:
  cloud: aws
  region: us-east-1
  image_id: ami-0d8c624d9d0f9af69
  # or:
  # image_id:
  #   us-east-1: ami-0d8c624d9d0f9af69
  #   us-west-1: ami-0257857e3e25bf4d8

We now show:

» sky launch ~/test.yaml
Task from YAML spec: /Users/zongheng/test.yaml
W 06-14 14:31:27 resources.py:486] Request AWS(m6i.2xlarge, image_id={'us-east-1': 'ami-0d8c624d9d0f9af69'}) cannot be satisfied by any feasible region. To fix, check that ssh_proxy_command's region keys include the regions to use.
I 06-14 14:31:27 optimizer.py:1009] No resource satisfying AWS(image_id={'us-east-1': 'ami-0d8c624d9d0f9af69'}) on AWS.
...

Tested (run the relevant ones):

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

[Michaelvll]

Thanks for adding this @concretevitamin! LGTM.
Comment: Seems we now removed the warning for the image_id, but image_id can fail if the user brings some corner-case images, such as without rsync installed or having some incompatible package pre-installed. I am ok with removing the warning for now and wait for feedback from the users.