For example, if "core/" isn't writeable, the updater fails during the recursive copy. If this is the case, we should only offer a 'download to desktop' option.
We can also revisit #1215 and , i.e., to generate robots.txt at installation, so even a manual installation won't overwrite a user's customized robots.txt.
See also #1153
updater: perhaps check all destination folders and files are writeable? (to avoid partial copy)