[Storage] Improve UX for error message when bucket is not accessible

[romilbhardwaj]

Changes the error message when a bucket is not accessible to focus on fix rather than the problem. Considered moving the root error (boto3/gcloud) to after our exception or removing it altogether, but decided it's better to let it come first, followed by our exception.

Before:

botocore.exceptions.ClientError: An error occurred (403) when calling the HeadBucket operation: Forbidden

The above exception was the direct cause of the following exception:

sky.exceptions.StorageBucketGetError: Failed to connect to an existing bucket 'mybucket'.
Please check if:
  1. the bucket name is taken and/or 
  2. the bucket permissions are not setup correctly. To debug, consider using aws s3 ls checkpoint.

After:

botocore.exceptions.ClientError: An error occurred (403) when calling the HeadBucket operation: Forbidden

The above exception was the direct cause of the following exception:

sky.exceptions.StorageBucketGetError: Failed to connect to an existing bucket 'mybucket'. This is likely because it is a private bucket you do not have access to.
To fix: 
  1. If you are trying to create a new bucket - use a different name.
  2. If you are trying to connect to an existing bucket - make sure you have access to it. To debug, consider running `aws s3 ls mybucket`.

Tested (run the relevant ones):

• Test with a common bucket name like checkpoint for s3, gcs and r2.
• Relevant individual smoke tests: pytest tests/test_smoke.py::TestStorageWithCredentials

[concretevitamin]

Thanks @romilbhardwaj. A few nits.

[romilbhardwaj]

Done, thanks! Merging now.