fix: if reference exceeds the threshold return 400 and detail

If the reference in the API request exceeds the threshold allowed by the
reference package (NOTE: this isn't defined by distribution
specification!) we return 500 back to the client.

This commit makes sure we return 400 and the explanation of the error in
the returned JSON payload.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/api/errcode/errors.go

@@ -224,11 +224,20 @@ func (errs Errors) MarshalJSON() ([]byte, error) {
 			msg = err.Code.Message()
 		}
 
-		tmpErrs.Errors = append(tmpErrs.Errors, Error{
+		tmpErr := Error{
 			Code:    err.Code,
 			Message: msg,
 			Detail:  err.Detail,
-		})
+		}
+
+		// if the detail contains error extract the error message
+		// otherwise json.Marshal will not serialize it at all
+		// https://github.com/golang/go/issues/10748
+		if detail, ok := tmpErr.Detail.(error); ok {
+			tmpErr.Detail = detail.Error()
+		}
+
+		tmpErrs.Errors = append(tmpErrs.Errors, tmpErr)
 	}
 
 	return json.Marshal(tmpErrs)