extension/src: show output channel upon failure

CL 559738 changes the type of output channel to log output channel.
The log output channel does not show by default.

The extension will only show the output channel upon failure, show
information message upon success.

For #3359

Change-Id: Id4bb8ff053748c80ac3192db34aed0c4c9c5acec
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/691615
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Madeline Kalil <mkalil@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: h9jiang

extension/src/goInstall.ts

@@ -60,11 +60,13 @@ export const installCurrentPackage: CommandFactory = () => async () => {
 
 	outputChannel.appendLine(`Installing ${importPath === '.' ? 'current package' : importPath}`);
 
-	cp.execFile(goRuntimePath, args, { env, cwd }, (err, stdout, stderr) => {
+	cp.execFile(goRuntimePath, args, { env, cwd }, (err, _, stderr) => {
 		if (err) {
 			outputChannel.error(`Installation failed: ${stderr}`);
+			outputChannel.show();
 		} else {
 			outputChannel.appendLine('Installation successful');
+			vscode.window.showInformationMessage('Installation successful');
 		}
 	});
 };