rfc5789 defines response codes returned by server for several situations:

1) Malformed patch document. When the server determines that the patch document provided by the client is not properly formatted, it SHOULD return a 400 (Bad Request) response.
Current response - 500
Test: https://github.com/resteasy/Resteasy/pull/1437/files#diff-ad7e017dfe2da2f8930e668c932e1e3bR63

2) Client sends valid Patch request with with valid format descriptor, but the resource doesn't exists. Server should return 404 (Not found).
Current response - 500
Test: https://github.com/resteasy/Resteasy/pull/1437/files#diff-ad7e017dfe2da2f8930e668c932e1e3bR96

3) Client sends Patch request to patch property of the object which doesn't exists. Server should return 409 (Conflict).
Current response - 500
Test: https://github.com/resteasy/Resteasy/pull/1437/files#diff-ad7e017dfe2da2f8930e668c932e1e3bR116

Note: Please remove '@Category(ExpectedFailing.class)' line from the tests, once it is fixed.