fix: implement RelationError, improve unique relations validation

[kettanaito]

• Closes Describe approach of error handling #307
• Documents error handling in README.md.
• Implements a RelationError class.
• Refactors the OperationError to include code instead of specific operation names.
• Validates unique relations for the lack of other owners referencing the same foreign record(s) (for one-way unique relations).
• Adds missing tests for unique relations.

Todos

• Use code for OperationError instead of .operationName. Make it an enum.
• Change the tests to assert the right errors are being thrown.
• Add unique many-to relation tests.
  • one-to-many unique relations tests are present. The many-to relations cannot be unique by design; they implied that many records may reference many other records.

[kettanaito]

Released: v1.0.1 🎉

This has been released in v1.0.1.

• 📄 Release notes
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