git-gui: Display message box when we cannot find git in $PATH

git-gui: Display message box when we cannot find git in $PATH

If we cannot find the git executable in the user's $PATH then
we cannot function correctly.  Because we need that to get the
version so we can load our library correctly we cannot rely on
the library function "error_popup" here, as this is all running
before the library path has been configured, so error_popup is
not available to us.

Signed-off-by: Shawn O. Pearce <redacted>