git-am --resolved: more usable error message.

git-am --resolved: more usable error message.

After doing the hard work of hand resolving the conflicts in the
working tree, if the user forgets to run update-index to mark
the paths that have been resolved, the command gave an
unfriendly "fatal: git-write-tree: not able to write tree" error
message.  Catch the situation early and give more meaningful
message and suggestion.

Noticed and suggested by Len Brown.

Signed-off-by: Junio C Hamano <junkio@cox.net>