v2: Update validator

So far, validation in petab.v2 is too lax and validator errors aren't too pretty and are often truncated.

• Readable error message
• include sufficient context in error messages
• Check for missing columns in files
• Consider using strict mode for validating files, but be more relaxed when programmatically constructing PEtab objects
• Decide whether we need to be able to de-serialize (somewhat) incorrect PEtab problems

The issue identifies validation in petab.v2 but names no files or tests. Start by locating the v2 validator and its validation tests, then separate missing-column checks from the strict-mode and deserialization decisions. Done requires an agreed scope and completion of the selected checklist items without truncated or context-poor errors.