git-difftool--helper.sh: exit upon initialize_merge_tool errors

git-difftool--helper.sh: exit upon initialize_merge_tool errors

Since the introduction of 'initialize_merge_tool' in de8dafbada
(mergetool: break setup_tool out into separate initialization function,
2021-02-09), any errors from this function are ignored in
git-difftool--helper.sh::launch_merge_tool, which is not the case for
its call in git-mergetool.sh::merge_file.

Despite the in-code comment, initialize_merge_tool (via its call to
setup_tool) does different checks than run_merge_tool, so it makes sense
to abort early if it encounters errors. Add exit calls if
initialize_merge_tool fails.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>