lib: fix invalid error for uri_content
It is not possible to universally call :path() on URIs as not all of them are of file scheme. Instead :uri() should have been called here to get URI representation.
This also not essentially downloading URI. It just receives it no matter what kind of URI it is so message about download is miss leading (although in most cases valid).
Fixes: https://gitlab.labs.nic.cz/turris/updater/updater/issues/293