Be more careful about download failures

Check that we were able to download all of the files
we need during the build.

Also move to a new mirror, since the previous server
no longer hosts version 5.1.

Change-Id: Ie388b1eebaebf67ba47f5513a2e6a79766e66c4d
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #8 from dhellmann/automate-build

Be more careful about download failures