text: Title: Switching to AWS account with insufficient permissions with AWS enabled crashes `sky launch` 
  Body: <!-- Describe the bug report / feature request here -->
  
  Repro
  - Have AWS, Azure, .. enabled
  - Now ` export AWS_SECRET_ACCESS_KEY=...` and `export AWS_ACCESS_KEY_ID=`, an account with insufficient permissions, e.g., no `ec2:DescribeRegions`
  - Run:
  ```
  » sky launch                                
  RuntimeError: Failed to retrieve AWS regions. Please ensure that the `ec2:DescribeRegions` action is enabled for your AWS account in IAM. Ref: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeRegions.html
  ```
  - Expected: shows other clouds in optimizer table; Actual: uncaught RuntimeError.
  
  
  <!-- If relevant, fill in versioning info to help us troubleshoot -->
  _Version & Commit info:_
  * `sky -v`: PLEASE_FILL_IN
  * `sky -c`: 71a95f4bf
  