Verify manifest is in paladin before trying to use it.

With increasing frequency the lumpy nightly test fails very early
because it ends up trying to get a manifest that only exists in
Android, not in ChromeOS.  This CL adds code to verify the manifest
version actually exists in the paladins list before trying to use it.

BUG=chromium:726778
TEST=Tested this code in the nightly build tests and it worked.

Change-Id: I86f8eb9c8e28fd4a404bc63d49d8d9fb27de8b3b
Reviewed-on: https://chromium-review.googlesource.com/517266


Commit-Ready: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>