make get-pip.py download failures more clear

curl dying ends up being a really unclear failure condition, and
hard to fingerprint in the gate. We should make this much more
explicit when we die.

Also, don't trust the upstream filename, because all the rest of
our logic would break if it changes anyway.

Change-Id: Ibc2a96b33471d24c597af0d7af896fb10523156f
Update again and retry a failed package install

On ubuntu, if we run into an error installing the package, it can be
because of transient upstream errors with repo sync state. Although
retrying the update won't fix it all of the time, it's low cost enough
to be worth a try before we totally give up.

Related-bug: 1286818
Change-Id: I522ac0d0bd8f82dc98f386c89f66c2b743efa525
Merge "Update again and retry a failed package install"
Merge "make get-pip.py download failures more clear"
Merge branch 'master' of github.com:openstack-dev/devstack