git-mergetool--lib.sh: add error message if 'setup_user_tool' fails

In git-mergetool--lib.sh::setup_tool, we check if the given tool is a
known builtin tool, a known variant, or a user-defined tool by calling
setup_user_tool, and we return with the exit code from setup_user_tool
if it was called. setup_user_tool checks if {diff,merge}tool.$tool.cmd
is set and quietly returns with an error if not.

This leads to the following invocation quietly failing:

git mergetool --tool=unknown

which is not very user-friendly. Adjust setup_tool to output an error
message before returning if setup_user_tool returned with an error.

Note that we do not check the result of the second call to
setup_user_tool in setup_tool, as this call is only meant to allow users
to redefine 'cmd' for a builtin tool; it is not an error if they have
not done so.

Note that this behaviour of quietly failing is a regression dating back
to de8dafbada (mergetool: break setup_tool out into separate
initialization function, 2021-02-09), as before this commit an unknown
mergetool would be diagnosed in get_merge_tool_path when called from
run_merge_tool.

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