Add UnavailableError and keep the PermissionError name

Host not connected, host shutting down and instance not running are
now UnavailableError (503) instead of ConflictError. The request error
migration notes move to a new 2.1 guide, since this ships in 2.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/controller/InstanceManager.test.js

@@ -172,7 +172,7 @@ describe("controller/InstanceManager", function () {
 		it("should throw if host not connected", async function () {
 			await assert.rejects(
 				() => instances.assignInstance(1, 5),
-				lib.ConflictError
+				lib.UnavailableError
 			);
 		});
 
@@ -204,7 +204,7 @@ describe("controller/InstanceManager", function () {
 
 			await assert.rejects(
 				() => instances.assignInstance(1, 6),
-				lib.ConflictError
+				lib.UnavailableError
 			);
 
 			assert.equal(unassignRequest, undefined);