improve url download failure display

improve url download failure display

* When downloading urls fail, explain which urls failed for which
  reasons.
* web: Avoid displaying a warning when downloading one url failed
  but another url later succeeded.

Some other uses of downloadUrl use urls that are effectively internal use,
and should not all be displayed to the user on failure. Eg, Remote.Git
tries different urls where content could be located depending on how the
remote repo is set up. Exposing those urls to the user would lead to wild
goose chases. So had to parameterize it to control whether it displays urls
or not.

A side effect of this change is that when there are some youtube urls
and some regular urls, it will try regular urls first, even if the
youtube urls are listed first. This seems like an improvement if
anything, but in any case there's no defined order of urls that it's
supposed to use.

Sponsored-by: Dartmouth College's Datalad project