Handle CostExplorer not being enabled in GET /cost-monitoring

[mendaomn]

Description

This PR handles the case in which Cost Explorer has not been manually launched by the customer, by having the GET /cost-monitoring endpoint return active: false, instead of returning an error

Changes

• update is_active() to that it returns False in case of a is_costexplorer_not_active_exception

How Has This Been Tested?

• manually, by assuming credentials of an account where Cost Explorer is not enabled and verifying the client successfully returns False
  • verified that, before the fix, it raises an Exception
• unit tests

PR Quality Checklist

• [x] I added tests to new or existing code
• [ ] I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
• [ ] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
• [ ] I made sure that any GitHub issue solved by this PR is correctly linked
• [ ] I checked that infrastructure/update_infrastructure.sh runs without any error
• [ ] I checked that npm run build builds without any error
• [ ] I checked that clusters are listed correctly
• [ ] I checked that a new cluster can be created (config is produced and dry run passes)
• [ ] I checked that login and logout work as expected

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