AWS image ID: IndexError: list index out of range

[concretevitamin]

commit 9d05415

Repro (from a user, so this is likely a private image)

resources:
 any_of:
    - cloud: aws
      region: us-west-2
      image_id:
        us-west-2: ami-0514bcd0cfc2aa8e2
      cpus: 2+
      memory: 4+
      disk_size: 50
      use_spot: true

sky launch test.yaml shows

...
  File "/Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/resources.py", line 1223, in _from_yaml_config_single
    return Resources(**resources_fields)
  File "/Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/resources.py", line 206, in __init__
    self._try_validate_image_id()
  File "/Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/resources.py", line 819, in _try_validate_image_id
    image_size = self.cloud.get_image_size(image_id, region)
  File "/Users/zongheng/Dropbox/workspace/riselab/sky-computing/sky/clouds/aws.py", line 262, in get_image_size
    image_info = image_info['Images'][0]
IndexError: list index out of range

Changing AMI id to -1234 shows this error correctly:

ValueError: Image 'ami-1234' not found in AWS region us-west-2.

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