BOLT 1: Restore all-zero error semantics

There were valid uses for "I don't want to talk to you anymore" apparently!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

01-messaging.md

@@ -318,6 +318,8 @@ For simplicity of diagnosis, it's often useful to tell a peer that something is
 
 #### Requirements
 
+The channel is referred to by `channel_id`, unless `channel_id` is 0 (i.e. all bytes are 0), in which case it refers to all channels.
+
 The funding node:
   - for all error messages sent before (and including) the `funding_created` message:
     - MUST use `temporary_channel_id` in lieu of `channel_id`.
@@ -330,7 +332,7 @@ A sending node:
   - SHOULD send `error` for protocol violations or internal errors that make channels unusable or that make further communication unusable.
   - SHOULD send `error` with the unknown `channel_id` in reply to messages of type `32`-`255` related to unknown channels.
   - when sending `error`:
-    - MUST fail the channel referred to by the error message.
+    - MUST fail the channel(s) referred to by the error message.
   - when sending `warning`:
 	- MAY set `channel_id` to all zero if the warning is not related to a specific channel.
   - MAY close the connection after sending.