Revert "Removing a timer that isn't running is a success."

This reverts commit 795499a529.

This breaks compatibility with SDL2 and there isn't a compelling reason to make this change.

src/timer/SDL_timer.c

@@ -353,6 +353,7 @@ int SDL_RemoveTimer(SDL_TimerID id)
 {
     SDL_TimerData *data = &SDL_timer_data;
     SDL_TimerMap *prev, *entry;
+    SDL_bool canceled = SDL_FALSE;
 
     if (!id) {
         return SDL_InvalidParamError("id");
@@ -376,10 +377,15 @@ int SDL_RemoveTimer(SDL_TimerID id)
     if (entry) {
         if (!SDL_AtomicGet(&entry->timer->canceled)) {
             SDL_AtomicSet(&entry->timer->canceled, 1);
+            canceled = SDL_TRUE;
         }
         SDL_free(entry);
     }
-    return 0;
+    if (canceled) {
+        return 0;
+    } else {
+        return SDL_SetError("Timer not found");
+    }
 }
 
 #else