Wayland: Fix error for missing protocol

A protocol not being available is not a platform error.  The platform is
doing fine; we're just sad that it lacks a feature we want.

src/wl_window.c

@@ -2507,8 +2507,8 @@ static void lockPointer(_GLFWwindow* window)
 {
     if (!_glfw.wl.relativePointerManager)
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
-                        "Wayland: no relative pointer manager");
+        _glfwInputError(GLFW_FEATURE_UNAVAILABLE,
+                        "Wayland: The compositor does not support pointer locking");
         return;
     }