Support Unmarshaling of Errors

[DQSevilla]

Is your feature request related to a problem? Please describe.
Currently our lib does not support Unmarshaling errors because it is not seen in requests to JSON:API servers, but it would be a nice-to-have feature for testing / consuming responses from JSON:API-servers.

Describe the solution you'd like
Unmarshal works for jsonapi.Error

Describe alternatives you've considered
N/A

Additional context
There are other things that this lib does not Unmarshal, like links or relationship metas, that may be interesting to do as well

[johnemau]

This would be nice to have, as a workaround I am doing the following:

import (
  "encoding/json"
  "github.com/DataDog/jsonapi"
)

// ...

// https://github.com/DataDog/jsonapi/issues/36
type jsonAPIDocumentWithErrors struct {
	Errors []*jsonapi.Error `json:"errors"`
}

// ....

var errorDoc jsonAPIDocumentWithErrors
_ = json.Unmarshal(bytes, &errorDoc)

// check errorDoc.Errors