Move a ton of Channel functions to ChannelError from HandleError

This is a big patch, but its all very mechanical, everything here
should be pretty obvious, and it all has to happen at once due to a
few common utility functions all having the same return type.

Note that this exposes a race in channel closure where we may
access a channel via some non-peer-specific mechanism like
forwarding an HTLC or sending a payment during the time between
the channel gave us a Close error and expected us to never call it
again and the time we actually removed it from the channel_state
set outside of the internal_* handler.

Author: TheBlueMatt