Retry button in upload failure message window

[mstock]

Especially in places where internet connectivity isn't ideal and uploads often fail due to e.g. timeouts, it would save some steps if the window which shows this error had something like a 'Retry Upload' button which would perform a retry, maybe after checking if the changeset is actually still open and empty.

This seems to be related to #2617 or at least could make use of that functionality.