Log 'ip route show table' errors to corresponding error file

Log 'ip route show table' errors to corresponding error file

We are iterating over the defined tables in /etc/iproute2/rt_tables,
though for tables that don't exist, ip reports an ugly error message,
that's a bit confusing during execution within grml-hwinfo:

| root@grml ~ # grep -v '^#' /etc/iproute2/rt_tables
| 255     local
| 254     main
| 253     default
| 0       unspec
| root@grml ~ # ip route show table 253
| Error: ipv4: FIB table does not exist.
| Dump terminated

Instead, store its error message to an according error file.