Bug 1623350 - Convert exited response to an error. r=jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D67456

--HG--
extra : moz-landing-system : lando

devtools/docs/backend/protocol.md

@@ -672,7 +672,7 @@ where *tabThreadActor* is the name of a thread-like actor representing the tab's
 If the user closes the tab before the client attaches to it, *targetActor* replies:
 
 ```
-{ "from":<targetActor>, "type":"exited" }
+{ "from":<targetActor>, "error":"exited" }
 ```
 
 When the client is no longer interested in interacting with the tab, the client can request:

devtools/server/actors/targets/browsing-context.js

@@ -6,6 +6,10 @@
 
 /* global XPCNativeWrapper */
 
+// protocol.js uses objects as exceptions in order to define
+// error packets.
+/* eslint-disable no-throw-literal */
+
 /*
  * BrowsingContextTargetActor is an abstract class used by target actors that hold
  * documents, such as frames, chrome windows, etc.
@@ -988,7 +992,9 @@ const browsingContextTargetPrototype = {
 
   attach(request) {
     if (this.exited) {
-      return { type: "exited" };
+      throw {
+        error: "exited",
+      };
     }
 
     this._attach();