added error codes for pagination

Signed-off-by: eyjhb <eyjhbb@gmail.com>

registry/api/v2/descriptors.go

@@ -490,6 +490,18 @@ var routeDescriptors = []RouteDescriptor{
 							},
 						},
 						Failures: []ResponseDescriptor{
+							{
+								Name:        "Invalid pagination number",
+								Description: "The received parameter n was invalid in some way, as described by the error code. The client should resolve the issue and retry the request.",
+								StatusCode:  http.StatusBadRequest,
+								Body: BodyDescriptor{
+									ContentType: "application/json",
+									Format:      errorsBody,
+								},
+								ErrorCodes: []errcode.ErrorCode{
+									ErrorCodePaginationNumberInvalid,
+								},
+							},
 							unauthorizedResponseDescriptor,
 							repositoryNotFoundResponseDescriptor,
 							deniedResponseDescriptor,

registry/api/v2/errors.go

@@ -133,4 +133,14 @@ var (
 		longer proceed.`,
 		HTTPStatusCode: http.StatusNotFound,
 	})
+
+	// ErrorCodePaginationNumberInvalid is returned when the `n` parameter is
+	// not an integer, or `n` is negative.
+	ErrorCodePaginationNumberInvalid = errcode.Register(errGroup, errcode.ErrorDescriptor{
+		Value:   "PAGINATION_NUMBER_INVALID",
+		Message: "invalid number of results requested",
+		Description: `Returned when the "n" parameter (number of results
+		to return) is not an integer, or "n" is negative.`,
+		HTTPStatusCode: http.StatusBadRequest,
+	})
 )