[PATCH] Explain what went wrong on update-cache of new file

If somebody tries to run `git update-cache foo', where foo is a new
file, git dies with a rather cryptic error message:

 fatal: Unable to add foo to database

This trivial patch makes git explain what probably went wrong.  It is
not a perfect diagnosis of all error paths, but for 90% of the cases it
should provide the user with the clue they need.

[jc: I ended up wording slightly differently, and fixed another
 confusing error message I noticed while reviewing the code.]

Signed-off-by: Amos Waterland <apw@rossby.metr.ou.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>