Consider support error type

[poef]

Many online systems require non ambiguous errors that are also machine readable. See for example: https://www.rfc-editor.org/rfc/rfc7807.html
If jsontag had such a type, this would ease parsing error results.

Errors often have extra details, so it should extend the JSON object type, e.g:

<error>{
  "message": "The error message"
}

The error object may be extended with extra properties, only the "message" property is required.

This will be the first tag/type to extend the object type, so it will impact the parser code a bit more than other types.