fix: improve wording for unassignable Policy

[romanblanco]

When Compliance tries to assign Host to a Policy, it returns either 202 in case of success or 404 in any other case.

When one tries to assign a Host that already has a Policy assigned, the Policy is not anymore in the list of available options, hence the process can not succeed with creating the link and returns 404.

That in the logic of insights-client seems as failure, and it returns non-zero error status with the confusing message "Policy ... does not exist".

This commit is to improve the message sent in such case to make it less misleading.

[LightOfHeaven1994]

@romanblanco as there are might be 2 cases:

1. Policy doesn't exist
2. Policy exists but host can't be assigned due to already existing association

For the 2nd case we might want to add additional request to else block: systems/{ID}/policies?limit=100&ids_only=true and check if policy id is already there - then we return message "Policy already associated to host"
And if not - "Policy doesn't exist"

It's not cool to have another request for that but it will be beneficial for all of us + this request is easy one for backend.

You also need to sign off your PR (git commit --amend -s) and update tests

[Mishrasubha]

can we also change Operation completed successfully to something more specific like System successfully assigned to policy?

[xiangce]

@romanblanco, Please also update the expected assert string in tests. Thanks.