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.

extensions/notebook/src/jupyter/jupyterServerManager.ts

@@ -62,7 +62,7 @@ export class LocalJupyterServerManager implements nb.ServerManager, vscode.Dispo
 			this._onServerStarted.fire();
 
 		} catch (error) {
-			this.apiWrapper.showErrorMessage(localize('startServerFailed', 'Starting local Notebook server failed with error: {0}', utils.getErrorMessage(error)));
+			// this is caught and notified up the stack, no longer showing a message here
 			throw error;
 		}
 	}