feat(asset): fail when an asset doesn't exist

birb/core/assets.lua

@@ -67,10 +67,17 @@ function AssetManager:clearLocal()
 end
 
 function AssetManager:get(name)
+    local asset
     if self.locals[name] ~= nil then
-        return self.locals[name]
+        asset = self.locals[name]
+    end
+    asset = self.globals[name]
+
+    if (asset ~= nil) then
+        return asset
+    else
+        core.debug:fail("birb.modules.assets", "L'asset " .. name .. " n'existe pas.")
     end
-    return self.globals[name]
 end
 
 -- Specific functions