Change some incorrect error types in proxy stores from API errors to

distribution errors.  Fill in missing checks for mutations on a registry pull-through
cache.  Add unit tests and update documentation.

Also, give v2.ErrorCodeUnsupported an HTTP status code, previously it was
defaulting to 500, now its 405 Method Not Allowed.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/api/errcode/register.go

@@ -30,7 +30,7 @@ var (
 		Message: "The operation is unsupported.",
 		Description: `The operation was unsupported due to a missing
 		implementation or invalid set of parameters.`,
-		HTTPStatusCode: http.StatusBadRequest,
+		HTTPStatusCode: http.StatusMethodNotAllowed,
 	})
 
 	// ErrorCodeUnauthorized is returned if a request is not authorized.