add `loc` and `file_position` to `PydanticValueError`

[samuelcolvin]

More changes after #185.

Also, pydantic/pydantic#4254.

We should add loc to PydanticValueError which gets appended to the error loc

Also add file_position (tuple[int, int] of (line, col)), one day when we have a custom JSON parser we can populate this in pydantic-core, until then we just add it via PydanticValueError.

file_position will require some pretty output in error messages.