[AWS] Fix fetching AZ when describe zones permission does not exist in all regions

[Michaelvll]

Fixes #2451

Current logging for sky check:

$ sky check                                                                                                                                                                                          
Checking credentials to enable clouds for SkyPilot.
AWS: [WARNING] Missing availability zone mappings for the following enabled regions:
Regions                                                       Reason                                                                            
['us-east-1', 'ca-central-1', 'ap-southeast-2', 'ap-          Failed to retrieve availability zones. Please ensure that the                     
northeast-2', 'ap-northeast-1', 'ap-south-1', 'eu-north-1',   `ec2:DescribeAvailabilityZones` action is enabled for your AWS account in IAM.    
'us-west-2', 'eu-west-1', 'eu-west-3', 'us-east-2', 'ap-      Ref: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeAvailabi  
northeast-3', 'eu-central-1', 'ap-southeast-1', 'eu-west-2']  lityZones.html.                                                                   
  AWS: enabled          
  Azure: enabled          
  GCP: enabled          
  IBM: enabled          
  Kubernetes: enabled          
  Lambda: enabled          
  OCI: enabled          
  SCP: enabled          
  Cloudflare (for R2 object store): enabled 

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky check with an AWS account with the following IAM policy (only allow the permission for a single region:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Deny",
            "Action": "ec2:DescribeAvailabilityZones",
            "Resource": "*"
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": "ec2:DescribeAvailabilityZones",
            "Resource": "*",
            "Condition": {
                "StringEquals": {
                    "aws:RequestedRegion": "us-west-1"
                }
            }
        }
    ]
}

• 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

[TRT-BradleyB]

This works for me now! re. #2451

[concretevitamin]

Thanks for fixing this @Michaelvll! Some minor comments.

[concretevitamin]

(not feeling strongly) nit: AwsAzFetchingError

https://google.github.io/styleguide/cppguide.html#General_Naming_Rules

[Michaelvll]

I slightly prefer the AWS, as the AWSNodeProvider from ray code already applies this, but feel free to push back.

[Michaelvll]

Latest UX:

$ sky check                                                                                                                
Checking credentials to enable clouds for SkyPilot.
AWS: [WARNING] Missing availability zone mappings for the following enabled regions:
Region                                                      Reason                                                                            
ap-southeast-2, ap-northeast-2, us-west-2, ap-south-1,      Failed to retrieve availability zones. Please ensure that the                     
us-east-2, eu-west-2, eu-west-1, eu-central-1, us-east-1,   `ec2:DescribeAvailabilityZones` action is enabled for your AWS account in IAM.    
ap-northeast-1, ap-northeast-3, eu-west-3, ap-southeast-1,  Ref: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeAvailabi  
eu-north-1, ca-central-1                                    lityZones.html.                                                                   
  AWS: enabled   

[concretevitamin]

LGTM, thanks @Michaelvll.

[concretevitamin]

How about just adhere to the name, AUTH_FAILURE?