Redirect `ip mrule show` errors to ip_mrule.error

Redirect `ip mrule show` errors to ip_mrule.error

`ip mrule show` depends on CONFIG_IP_MROUTE_MULTIPLE_TABLES
for IPv4 and CONFIG_IPV6_MROUTE_MULTIPLE_TABLES for IPv6,
if either is missing then it fails:

  % ip -6 mrule show
  32767:  from all lookup main
  % ip -4 mrule show
  RTNETLINK answers: Address family not supported by protocol
  Dump terminated
  % ip mrule show
  RTNETLINK answers: Address family not supported by protocol
  Dump terminated

  % grep MROUTE /boot/config-$(uname -r)
  CONFIG_IP_MROUTE=y
  # CONFIG_IP_MROUTE_MULTIPLE_TABLES is not set
  CONFIG_IPV6_MROUTE=y
  CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
  % uname -r
  4.13.13-6-pve

Noticed on a customer system running Proxmox kernel,
reported towards upstream: https://bugzilla.proxmox.com/show_bug.cgi?id=1711