Replaced PreconditionFailedException with UnprocessableEntityException in RepositoryValidatingInterceptor. This aligns exception usage with the appropriate HTTP status code for validation errors, eg 422 - Unprocessable Entity.

From the documentation:

3.1.0.4.2 Rejecting Updates
Servers are permitted to reject update interactions because of integrity concerns or other business rules, and return HTTP status codes accordingly (usually a 422). Note that there are potential security issues relating to how rejections are handled. See the security page for more information.

Common HTTP Status codes returned on FHIR-related errors (in addition to normal HTTP errors related to security, header and content type negotiation issues):

• 400 Bad Request - resource could not be parsed or failed basic FHIR validation rules (or multiple matches were found for conditional criteria)
• 401 Not Authorized - authorization is required for the interaction that was attempted
• 404 Not Found - resource type not supported, or not a FHIR end-point
• 405 Method Not allowed - the resource did not exist prior to the update, and the server does not allow client defined ids
• 409/412 - version conflict management - see below
• 422 Unprocessable Entity - the proposed resource violated applicable FHIR profiles or server business rules