read-cache.c: mark more strings for translation

There are a couple other improvements on these strings as well:

- add missing colon (as separator)
- quote paths
- provide more information on error messages
- keep first word in lowercase

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>