Clear error for incomplete property definitions

Commit message

[FIX] orm: validate the required keys of a property definition first

`_validate_properties_definition` checks the property name before it
checks that the required keys are present, so a definition without
`name` raises `KeyError: 'name'` instead of the `ValueError` the method
documents, and the "Some key are missing [name]" branch is unreachable.

Swapping the two checks lets the callers (`create`, `write`, or the
import converter) report the error instead of crashing on it.