Resolves CCD-1635

This PR addresses ...
Running the crds_s3_get script without first installing the aws-related optional dependencies (pip install crds['aws']) produces a traceback that does not make clear what actually is causing the error. The script has been updated to first check to make sure the necessary libraries are installed and produces a helpful explanation if either awscli or boto3 are missing when the error is caught.

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)