open_proc: don't print error twice

open_proc* and fopen_proc* are all macros, and they print errors in case
something is wrong (and as they are macros they print correct file:line
info, too). So, there is no need to print another error message
right after calling those.

This fixes double error messages like these:

(00.047295) Error (criu/proc_parse.c:990): Can't open 18472/status on procfs: No such file or directory
(00.047298) Error (criu/proc_parse.c:992): Can't open proc status: No such file or directory

travis-ci: success for Assorted nitpicks
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>