Raise a setup warning for an unparseable VCS binary version

Summary:
Hit this locally, with an error like:

Version <empty string> is older than 1.9, the minimum supported version.

(Where <empty string> was just the empty string, not literally the text <empty string>.)

Be more careful about parsing versions, and parse the newer string.

Test Plan: Got "unknown version" with intentionally-broken test data, then clean readout.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11558