cros_sdk: check for certificate errors, give appropriate message
cros_sdk: check for certificate errors, give appropriate message

Also, provides a --no-check-certificate option that allows overriding
the behaviour.

Note that this also silences the stderr from the first 'probe' curl
run, eliminating the first and useless curl output.

BUG=chromium-os:25517
TEST=cros_sdk --chroot x --replace (and see it work)
TEST=manually provide an incorrect command and check its handling

Change-Id: Icf048366ade67dde003723c374e5207a8b2ce7a6
Reviewed-on: https://gerrit.chromium.org/gerrit/19643
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

zhang0137/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite