Merge branch 'pb/mergetool-errors'

End-user experience of "git mergetool" when the command errors out has been improved. * pb/mergetool-errors: git-difftool--helper.sh: exit upon initialize_merge_tool errors git-mergetool--lib.sh: add error message for unknown tool variant git-mergetool--lib.sh: add error message if 'setup_user_tool' fails git-mergetool--lib.sh: use TOOL_MODE when erroring about unknown tool completion: complete '--tool-help' in 'git mergetool'