json-api error format

Part of #11.

types/validation-error.go

@@ -2,7 +2,23 @@ package types
 
 import "encoding/json"
 
-type ValidationError map[string][]string
+type Source struct {
+	Pointer string `json:"pointer"`
+}
+
+type ErrorDetail struct {
+	Source `json:"source"`
+	Detail string `json:"detail"`
+}
+
+func NewValidationError(attr, message string) ErrorDetail {
+	return ErrorDetail{
+		Source: Source{Pointer: "data/attributes/" + attr},
+		Detail: message,
+	}
+}
+
+type ValidationError []ErrorDetail
 
 func (v ValidationError) Error() string {
 	errs, err := json.Marshal(struct {