Fix #4029 Ensure changeKernels always resolves, even in error states (#4488)

* Fix #4029 Ensure changeKernels always resolves, even in error states
This is necessary to unblock reverting the kernel on canceling  Python install
- startSession now correctly sets up kernel information, since a kernel is loaded there.
- Remove call to change kernel on session initialize. This isn't needed due to refactor
- Handle kernel change failure by attempting to fall back to old kernel
- ExtensionHost $startNewSession now ensures errors are sent across the wire.
- Update AttachTo and Kernel dropdowns so they handle kernel being available. This is needed since other changes mean the session is likely ready before these get going

* Fix to handle python cancel and load existing scenarios
- Made changes to handle failure flow when Python dialog is canceled
- Made changes to handle initial load fail. Kernel and Attach To dropdowns show No Kernel / None and you can choose a kernel
- Added error wrapping in ext host so that string errors make it across and aren't lost.

src/sql/parts/notebook/notebook.component.ts

@@ -248,7 +248,7 @@ export class NotebookComponent extends AngularDisposable implements OnInit, OnDe
 		this._model = this._register(model);
 		this.updateToolbarComponents(this._model.trustedMode);
 		this._modelRegisteredDeferred.resolve(this._model);
-		await model.startSession(this.model.notebookManager);
+		await model.startSession(this.model.notebookManager, undefined, true);
 		this.detectChanges();
 	}