fix: handling timeouts with proper error messages

[diya-dhan]

Purpose

fix #916

When a timeout occurs and the subscription hasn't reached the target state, the plugin would crash without providing a readable error message. While the logic for generating a relevant error is already in place, the absence of a check for "nil" responses causes the execution to terminate abruptly. This PR resolves the issue by adding the necessary check and enhancing the error message for better clarity.

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

Checklist for reviewer

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• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
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[lechnerc77]

General remark: If we fix this here, we should make sure that the handling in the other resoures that support a timeout is consistent:

• subaccount_service_instance -> looks good, but please cross check
• subaccount_environment_instance -> Create and Update is missing the return when waiting for the state change