correct handling of errors with source/pointer "/data"

[senid231]

http://jsonapi.org/format/#error-objects

• source: an object containing references to the source of the error, optionally including any of the following members:
  • pointer: a JSON Pointer [RFC6901] to the associated entity in the request document [e.g. "/data" for a primary data object, or "/data/attributes/title" for a specific attribute].
  • parameter: a string indicating which URI query parameter caused the error.

so source/pointer and source/parameter is a different things, and they should be handled in a different way

also error for pointer /data should be handled as base error for ActiveModel::Errors object