specs-go: Make ErrorResponse a Go error.

Signed-off-by: Atlas Kerr <atlaskerr@gmail.com>

specs-go/v1/error.go

@@ -16,11 +16,21 @@ package v1
 
 // ErrorResponse is returned by a registry on an invalid request.
 type ErrorResponse struct {
-	Errors []ErrorDetail `json:"errors"`
+	Errors []ErrorInfo `json:"errors"`
 }
 
-// ErrorDetail describes a server error returned from a registry.
-type ErrorDetail struct {
+// Error implements the Error interface.
+func (er *ErrorResponse) Error() string {
+	return "distribution: registry returned error"
+}
+
+// Detail returns an ErrorInfo
+func (er *ErrorResponse) Detail() []ErrorInfo {
+	return er.Errors
+}
+
+// ErrorInfo describes a server error returned from a registry.
+type ErrorInfo struct {
 	Code    string `json:"code"`
 	Message string `json:"message"`
 	Detail  string `json:"detail"`