make curl show http errors to stderr

* Run curl with -S, so HTTP errors are displayed, even when
  it's otherwise silent.
* When downloading in --json or --quiet mode, use curl in preference
  to wget, since curl is able to display only errors to stderr, unlike
  wget.

This does mean that downloadQuiet is only silent on stdout, not necessarily
on stderr, which affects a couple other calls of it. For example,
downloading the .git/config of a http remote may show an error message now,
perhaps with slightly suboptimal formatting due to other output.

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