Fix assign formation not found errors

[la4ezar]

Description
Currently, when assigning something that does not exist, the errors are misleading:

• "message": "Object not found [object=runtimeLabel]"
• "message": "Object not found [object=applicationLabel]"
  This PR fixes that.

Changes proposed in this pull request:

• Fix not-found errors
• Add unit tests

Pull Request status

• Implementation
• Unit tests
• Integration tests
• chart/compass/values.yaml is updated
• Mocks are regenerated, using the automated script

[kyma-bot]

@la4ezar: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pre-main-compass-gke-benchmark	15d2b58	link	false	/test pre-main-compass-gke-benchmark

Full PR test history. Your PR dashboard.

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[la4ezar]

/test pre-main-compass-integration-no-dump

[kyma-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ivantenevvasilev

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Needs approval from an approver in each of these files:

• OWNERS [ivantenevvasilev]
• components/director/internal/domain/formation/OWNERS [ivantenevvasilev]

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