Specify validation error JSON format

[nichtich]

Existing validation error "formats":

• https://ajv.js.org/api.html#error-objects
• https://www.w3.org/TR/shacl/#validation-report
• how about XML Schema validation errors?
• ...

Existing format:

• message mandatory error message
• error optional type of error, given as IRI
• position optional object mapping locator formats to locators

Possible additions:

• value : the invalid value

To be evaluated:

• rule (pointer to specific schematron rule, JSON schema element, SHACL shape...)
• sh:resultPath
• sh:severity (IRI)
• ...