gnrc_netapi: recover from message send errors

sys/net/gnrc/link_layer/netdev2/gnrc_netdev2.c

sys/net/gnrc/netapi/gnrc_netapi.c

sys/net/gnrc/network_layer/icmpv6/echo/gnrc_icmpv6_echo.c

sys/net/gnrc/network_layer/icmpv6/gnrc_icmpv6.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/network_layer/ndp/gnrc_ndp.c

sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c

...t/gnrc/network_layer/sixlowpan/frag/gnrc_sixlowpan_frag.c

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

sys/shell/commands/sc_icmpv6_echo.c

sys/shell/commands/sc_netif.c