Add Runtime error for attempt to establish Relationships to deleted Identities

[britsta]

Readiness checklist

• I added/updated tests.
• I ensured that the PR title is good enough for the changelog.
• I labeled the PR.
• I self-reviewed the PR.

Description

Currently, only the Backbone throws a generic error.platform.recordNotFound error when an Identity tries to create a Relationship based on a loaded RelationshipTemplate if the creator of the corresponding RelationshipTemplate has been deleted in the meantime.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...c/modules/relationships/RelationshipsController.ts	50.00%	2 Missing ⚠️
packages/transport/src/core/TransportCoreErrors.ts	87.50%	1 Missing ⚠️

Files with missing lines	Coverage Δ
packages/transport/src/core/TransportCoreErrors.ts	61.15% <87.50%> (+1.15%)	⬆️
...c/modules/relationships/RelationshipsController.ts	85.71% <50.00%> (-0.22%)	⬇️

[jkoenig134]

awesome!