Make error messages from cherry-pick/revert more sensible

Make error messages from cherry-pick/revert more sensible

The original "rewrite in C" did somewhat a sloppy job while
stealing code from git-write-tree.

The caller pretends as if the write_tree() function would return
an error code and being able to issue a sensible error message
itself, but write_tree() function just calls die() and never
returns an error.  Worse yet, the function claims that it was
running git-write-tree (which is no longer true after
cherry-pick stole it).

Tested-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>