Fix uploads getting stuck in 'processing' state (fix #3659).

Bug: if an upload timed out while downloading the file, Upload#process!
would catch the error and attempt to retry, but since the upload was
already in the 'processing' state, on the second try `process!` would
bail out immediately and leave the upload stuck in the 'processing' state.

Fix: remove the retry logic from Upload#process!. Let Downloads::File#download!
(which had its own retry logic) handle it instead.

test/unit/upload_test.rb

@@ -305,5 +305,15 @@ class UploadTest < ActiveSupport::TestCase
         assert_nothing_raised {@upload.process!}
       end
     end
+
+    context "on timeout errors" do
+      should "leave the upload in an error state" do
+        HTTParty.stubs(:get).raises(Net::ReadTimeout)
+        @upload = FactoryBot.create(:source_upload)
+        @upload.process!
+
+        assert_match(/\Aerror/, @upload.status)
+      end
+    end
   end
 end