While bisecting, or building a nightly Ruby CI and some other situations,
it is useful to be able to build a specific Ruby revision rather than
just the tip of Ruby's master branch.
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
