DEV: Update call sites using BaseStore#download but expecting exceptions (#21551)

In #21498, we split `BaseStore#download` into a "safe" version which returns nil on errors, and an "unsafe" version which raises an exception, which was the old behaviour of `#download`.

This change updates call sites that used the old `#download`, which raised exceptions, to use the new `#download!` to preserve behaviour (and silence deprecation warnings.)

It also silences the deprecation warning in tests.

app/models/optimized_image.rb

@@ -146,7 +146,7 @@ class OptimizedImage < ActiveRecord::Base
       if local?
         Discourse.store.path_for(self)
       else
-        Discourse.store.download(self).path
+        Discourse.store.download!(self).path
       end
     File.size(path)
   end