@dominic i try to avoid stating proposals unless it is something that i am ready to take action on - otherwise it is just a #somebodyshould.
but here goes:

• packet-stream substream errors that are not originated by the remote api (i.e. that indicate that the rpc stream is broken) should be wrapped into a single error type, like PacketStreamError, or otherwise have some specific flag set.
• muxrpc errors should have specific names like MethodNotFoundError and MethodPermissionError, or otherwise specific flags.
• intermediate code that handles these errors should either pass them through unmodified, or only add properties to them.