sparse checkout: show error messages when worktree shaping fails

sparse checkout: show error messages when worktree shaping fails

verify_* functions can queue errors up and to be printed later at
label return_failed. In case of errors, do not go to label "done"
directly because all queued messages would be dropped on the floor.

Found-by: Joshua Jensen <jjensen@workspacewhiz.com>
Tracked-down-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>