Documentation on error conventions

[TheTechmage]

While working on mediation-related code, we ran into some cases where an error occurred, but it's unknown who to respond back to due to forward messages being Anon-crypted. Because of our need to inform the sender about an error, it would be helpful to have some clarification on when it is/is not appropriate to send back error messages (and whether errors should be reported via plain text or not)

cc: @dbluhm