Improve error messages that impact newcomers

Discussion #704 highlights 3 situations in which Gatekeeper can provide better error messages to help newcomers:
1. Reporting that DPDK does not have a driver for a given NIC. The message should add the link to the list of [supported NICs](https://core.dpdk.org/supported/nics/);
2. Checking that the intersection between variables `front_ports` and `back_ports` is empty. If not empty, report the problem and direct users to the question [Why do Gatekeeper servers need two network interfaces?](https://github.com/AltraMayor/gatekeeper/wiki/FAQ#why-do-gatekeeper-servers-need-two-network-interfaces) on our FAQ.
3. Enforcing variable `back_mtu` to be greater than variable `front_mtu` so no encapsulated packets to the Grantor servers are dropped. The test should only be performed when running as a Gatekeeper server.

While item 1 has to be done in the C code, items 2 and 3 can be implemented in Lua at `lua/net.lua`.