sync: log conformance errors if they happen

also send them via API to see them on admin
do not fatal with simple PKs

For conformance errors, some models' primary key will be a single value and JSON deserialization will fail. Return serialized primary key if deserialization fails
Enhance `NationalMeasurementUnitView`

- show national measurement unit only if available in map (see `NationalMeasurementUnit`)
- use parenthesis `(` instead of brackets `[`
Merge pull request #197 from bitzesty/sync-conformance-errors

sync: log conformance errors if they happen
Merge pull request #200 from bitzesty/enhance-national-measurement-unit-view

Enhance `NationalMeasurementUnit` view