magit-git-insert: log errors to .git/magit-errors
declare 1.9.1 to be the minimal Git version

While I would like to move to 2.0, let's keep supporting 1.9.1, since it
is available as a backport for Debian stable.  (1.7.10.4, the version in
Debian stable itself, won't do unfortunately).

Note that you *can* use older versions but if you do then some undefined
features will break, often without any meaningful error messages.