git-mergetool--lib.sh: use TOOL_MODE when erroring about unknown tool

git-mergetool--lib.sh: use TOOL_MODE when erroring about unknown tool

In git-mergetool--lib.sh::get_merge_tool_path, we check if the chosen
tool is valid via valid_tool and exit with an error message if not. This
error message mentions "Unknown merge tool", even if the command the
user tried was 'git difftool --tool=unknown'. Use the global 'TOOL_MODE'
variable for a more correct error message.

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