git-gui: Correctly report failures from git-write-tree

git-gui: Correctly report failures from git-write-tree

If git-write-tree fails (such as if the index file is currently
locked and it wants to write to it) we were not getting the error
message as $tree_id was always the empty string so we shortcut
through the catch and never got the output from stderr.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>