pubsys: fix AWS SDK API calls and surface service error codes

[etungsten]

Issue number:

N/A

Description of changes:

    pubsys: fix put-parameter taking a empty list
    
    Ensure we pass None if the list of names we're passing is empty.

    pubsys: fix AWS SDK API calls
    
    The EC2 API call for modify-snapshot-attribute and
    modify-image-attribute errors on empty vecs. This passes 'None' for the
    list parameters if the list is empty.

    pubsys: surface AWS API service error code in error log
    
    This surfaces the actual AWS API service error code in the error log message.

Testing done:
@jpculp ran the tests:

• Ran pubsys directly to revoke user ids
• Ran pubsys directly to grant user ids
• Published ami with cargo make -e BUILDSYS_ARCH=x86_64 -e BUILDSYS_VARIANT=aws-k8s-1.24 ami
• Granted permissions to ami with cargo make -e BUILDSYS_ARCH=x86_64 -e BUILDSYS_VARIANT=aws-k8s-1.24 -e GRANT_TO_USERS=<> grant-ami

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jpculp]

• Ran pubsys directly to revoke user ids
• Ran pubsys directly to grant user ids
• Published ami with cargo make -e BUILDSYS_ARCH=x86_64 -e BUILDSYS_VARIANT=aws-k8s-1.24 ami
• Granted permissions to ami with cargo make -e BUILDSYS_ARCH=x86_64 -e BUILDSYS_VARIANT=aws-k8s-1.24 -e GRANT_TO_USERS=XXXXXXXXX grant-ami

[etungsten]

Push above changes the if .is_empty() { None } else { Some(...) } to then_some()

[jpculp]

Re-ran the same cargo make tests. LGTM!