write-tree: properly detect failure to write tree objects

write-tree: properly detect failure to write tree objects

Tomasz Fortuna reported that "git commit" does not error out properly when
it cannot write tree objects out.  "git write-tree" shares the same issue,
as the failure to notice the error is deep in the logic to write tree
objects out recursively.

Signed-off-by: Junio C Hamano <gitster@pobox.com>