ls-files: Give hints when errors happen.

ls-files: Give hints when errors happen.

Without this patch "git commit file.c file2.c" produces the not
so stellar output:

	error: pathspec 'file.c' did not match any.
	error: pathspec 'file2.c' did not match any.

With this patch, the output is changed to:

	error: pathspec 'file.c' did not match any file(s) known to git.
	error: pathspec 'file2.c' did not match any file(s) known to git.
	Did you forget to 'git add'?

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>