Define how to response with an error

The main difficulty here is that if we get a command with an invalid id, it's
unclear what to return in the id field. This chooses to return null in that case.

index.bs

@@ -126,7 +126,7 @@ Message = (
 )
 
 CommandResponse = (
-  id: uint,
+  id: uint?,
   ResponseData,
 )
 
@@ -142,7 +142,7 @@ Error = (
     "invalid argument"
   ),
   message: text,
-  stacktrace: text,
+  stacktrace?: text,
 )
 
 CommandResult = {
@@ -296,9 +296,9 @@ To <dfn>process a command</dfn> given |data|:
       the <code>method</code> property of |data|.
 
    1. If |command| is not in the [=set of all command names=], return an
-      [=Error=] with [=error code=] [=unknown command=].
+      [=error=] with [=error code=] [=unknown command=].
 
-   1. Return an [=Error=] with [=error code=] [=invalid argument=].
+   1. Return an [=error=] with [=error code=] [=invalid argument=].
 </div>
 
 Transport {#transport}
@@ -490,9 +490,9 @@ To <dfn>handle an incoming message</dfn> given a [=WebSocket connection=]
  5. Let |result| be the result of [=process a command|processing a
     command=] given |data|.
 
- 6. If |result| is an [=Error=], then [=respond with an error=] given
-    |connection|, |result|, and |parsed|["<code>id</code>"], and finally
-    return.
+ 6. If |result| is an [=error=], then [=respond with an error=] given
+    |connection|, |parsed|["<code>id</code>"], and |result|'s [=error code=],
+    and finally return.
 
  7. Let |response| be a new [=map=] with the following properties:
 
@@ -514,12 +514,30 @@ To <dfn>handle an incoming message</dfn> given a [=WebSocket connection=]
 
 <div algorithm>
 To <dfn>respond with an error</dfn> given a [=WebSocket connection=]
-|connection| and an [=error code=] |code|:
+|connection|, |command id| and |error code|:
 
- 1. Issue: Form a valid JSON |errorObject| given |code|.
+ 1. If |command id| is not a number, let |command id| be null.
 
- 2. [=Send a WebSocket message=] comprised of |errorObject| over
-    |connection|.
+ 1. Let |error data| be a new map matching the <code>Error</code> production in
+    the [=local end definition=], with the <code>error</code> field set to
+    |error code|, the <code>message</code> field set to an
+    implementation-defined string containing a human-readable definition of the
+    error that occured and the <code>stacktrace</code> field optionally set to
+    an implementation-defined string containing a stack trace report of the
+    active stack frames at the time when the error occurred.
+
+ 1. Set the value of <code>id</code> on |error data| to |command id|.
+
+ 1. Assert: |error data| matches the <code>CommandResponse</code> definition in
+    the [=local end definition=].
+
+ 1. Let |response| be the result of [=serialize JSON to bytes|serializing JSON
+    to bytes=] given |error data|.
+
+ 1. Let |serialized| be the result of [=serialize JSON to
+    bytes|serializing JSON to bytes=] given |response|.
+
+ 1. [=Send a WebSocket message=] comprised of |serialized|| over |connection|.
 
 </div>