Clean up pagination specification

Some missing descriptions and error code for tags pagination was cleaned up to
ensure clarity. Specifically, we ensure the request variations are named and
the proper error codes are included.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/api/v2/descriptors.go

@@ -398,6 +398,8 @@ var routeDescriptors = []RouteDescriptor{
 				Description: "Fetch the tags under the repository identified by `name`.",
 				Requests: []RequestDescriptor{
 					{
+						Name:        "Tags",
+						Description: "Return all tags for the repository",
 						Headers: []ParameterDescriptor{
 							hostHeader,
 							authHeader,
@@ -455,6 +457,7 @@ var routeDescriptors = []RouteDescriptor{
 						},
 					},
 					{
+						Name:            "Tags Paginated",
 						Description:     "Return a portion of the tags for the specified repository.",
 						PathParameters:  []ParameterDescriptor{nameParameterDescriptor},
 						QueryParameters: paginationParameters,
@@ -483,6 +486,30 @@ var routeDescriptors = []RouteDescriptor{
 								},
 							},
 						},
+						Failures: []ResponseDescriptor{
+							{
+								StatusCode:  http.StatusNotFound,
+								Description: "The repository is not known to the registry.",
+								Body: BodyDescriptor{
+									ContentType: "application/json; charset=utf-8",
+									Format:      errorsBody,
+								},
+								ErrorCodes: []errcode.ErrorCode{
+									ErrorCodeNameUnknown,
+								},
+							},
+							{
+								StatusCode:  http.StatusUnauthorized,
+								Description: "The client does not have access to the repository.",
+								Body: BodyDescriptor{
+									ContentType: "application/json; charset=utf-8",
+									Format:      errorsBody,
+								},
+								ErrorCodes: []errcode.ErrorCode{
+									ErrorCodeUnauthorized,
+								},
+							},
+						},
 					},
 				},
 			},