Make SDL_SetError and friends unconditionally return -1.

This lets us change things like this...

    if (Failed) {
        SDL_SetError("We failed");
        return -1;
    }

...into this...

    if (Failed) {
        return SDL_SetError("We failed");
    }


 Fixes Bugzilla #1778.

src/thread/generic/SDL_systhread.c

@@ -28,8 +28,7 @@
 int
 SDL_SYS_CreateThread(SDL_Thread * thread, void *args)
 {
-    SDL_SetError("Threads are not supported on this platform");
-    return (-1);
+    return SDL_SetError("Threads are not supported on this platform");
 }
 
 void