Merge branch 'pb/mergetool-errors'

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'