[router] Batch writing error handling

[JordiSubira]

Currently the BR discards the packet that cause an error on the WriteBatch operation https://github.com/scionproto/scion/blob/v0.11.0/router/dataplane.go#L959-L963. This was introduced by #4351 and discussed here. As discussed, it isn't trivial to handle any potential errors, but perhaps we can handle some, e.g., if we get this type of error err": "write udp [fd00:f00d:cafe::7f00:12]:31004: sendmmsg: network is unreachable"} we could handle it and generate the appropriate SCMP Destination Unreachable error which would be useful for the remote sender to understand that the SCION host she/he cannot use such a path to reach the destination host.

[jiceatscion]

One other SCMP error that we SHOULD return but don't: Packet Too Big
That's the only available way to discover MTU issues and we don't enable it.