Clarify error message when PKI decryption fails

[llamafilm]

When a node receives a PKI DM with want_ack that it cannot decrypt, it currently NAKs with NO_CHANNEL. This commonly happens when it still has the sender’s old public key after they rotated. This is confusing and is easy to misinterpret because it sounds like a channel/PSK problem.

I propose adding a new Routing.Error enum called PKI_DECRYPT_FAILED and using it in ReliableRouter in this case instead of NO_CHANNEL.

Thoughts?

[SpudGunMan]

I vote better legacy DM handler

[llamafilm]

I'm not sure what you mean? I'm talking about PKI DMs, not legacy DMs.