This connects the stderr of download utilities like curl and wget to the
original stderr of the process so that they can detect a terminal and
print progress bars to it.
Previously, if `http get` failed, the `download_tarball` function would
still continue since the ERR trap had no effect at that point.
Given a script in the form of `{ ... } || return 1`, the expressions
that are the part of the first group are not subject to ERR trap since
they are non-last in a chain of expressions.
However, since we still can't count on the ERR trap taking effect in
this phase, better just rewrite the function to manually abort.
Fixes#394
