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