Fix hang when downloads fail

The HTTP client wasn't closed and kept the VM alive, probably because
the body of an error response wasn't consumed.

Change-Id: I7534dea010d88d155d664891bb523f097a5cda4d
Reviewed-on: https://dart-review.googlesource.com/c/deps/+/365760
Reviewed-by: Martin Kustermann <kustermann@google.com>
Auto-Submit: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>