pkg: Die if make -V BRANCH fails

If build logic breaks such that we can't figure out which branch we're
on, we should error out rather than shipping a default (and probably
wrong) /etc/pkg/FreeBSD.conf

Approved by: re (cperciva)
Reviewed by: ivy
MFC after: 5 minutes
Suggested by: ivy
Differential Revision: https://reviews.freebsd.org/D53487

(cherry picked from commit 0837b2ebd570ea092353aa3c46e374f7a1533a1d)
(cherry picked from commit 1f528e32f5098c5aaca9bd2fb6473631c3831fbf)