cat,nl: fix handling of open errors

cat,nl: fix handling of open errors

$ cat -n does_not_exist; echo $?
cat: does_not_exist: No such file or directory
1

function                                             old     new   delta
print_numbered_lines                                 118     129     +11
nl_main                                              196     201      +5
cat_main                                             421     425      +4
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 20/0)               Total: 20 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>