files-reg: Print file name on error

In case if there some error happened better to
know which exactly file failed on open, for example

 | Error (files-reg.c:535): Can't open file /lib64/libgcc_s-4.4.7-20120601.so.1 on restore: No such file or directory

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>