fix: error handling of entitlement polling

[lechnerc77]

Purpose

• This PR fixes the handling of error scenarios when polling for a state change for the resource entitlement.

• Error that occurs looks basically like this if the entitlement fails:

  When applying changes to btp_subaccount_entitlement.entitlement-quotas["xxx"], 
   provider
   │ "provider["registry.terraform.io/sap/btp"]" produced an unexpected new value: .amount: was cty.NumberIntVal(1), but now  
   │ cty.NumberIntVal(0).

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code via automated test

go test ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

Other Information

see also PR #352

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[rui1610]

lgtm