[cipd] Try to bootstrap CIPD from scratch if selfupdate fails.

When updating the CIPD client to be identified by SHA256 hash, old clients (that
have no idea about SHA256) fail during 'selfupdate'.

We'll roll our SHA256 support in two stages:
  1. Deploy new client that understand SHA256 using its SHA1 name, so
     self-update from old clients works.
  2. Deploy same (or newer) client using its SHA256 name. This would work since
     the client doing the self-update already understands SHA256 at this point.

But we can't guarantee that ALL buildtools deployments will update through
stages (1) and (2) sequentially. Some of them may skip (1) and end-up directly
in (2), failing on 'selfupdate'.

This CL makes sure they can recover from this state by rebootstraping the client
from scratch (this works with SHA256 hashes).

Similar CL in Chromium's depot_tools:
https://chromium-review.googlesource.com/1149454

Change-Id: I6bb6cb9980ad536008260af74ddec90ded51dd22