Fixed error message when trying to create an OpenGLES2 renderer on macOS

Testing: testsprite2 --renderer opengles2

OpenGLES2 isn't available by default, and we want to see the error "Could not load EGL library"

src/video/cocoa/SDL_cocoawindow.m

@@ -2274,7 +2274,9 @@ Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window)
 
     /* Not recognized via CHECK_WINDOW_MAGIC */
     if (data == nil) {
-        return SDL_SetError("Window data not set");
+        /* Don't set the error here, it hides other errors and is ignored anyway */
+        /*return SDL_SetError("Window data not set");*/
+        return -1;
     }
 
     /* NSWindow.screen may be nil when the window is off-screen. */