src/goLanguageServer: report when falling back to default providers

When language server is enabled but we could not start the language
server (e.g. because gopls isn't installed), report that in the Go
output channel before falling back to the default providers.

Considered notifications or updating the status bar. But when
this unfortunate fallback happens, the extension has already
triggered one or more notifications about the root cause,
so I decided to take this easy path.

Fixes #1019

Change-Id: I27d012d0f5caaeba83b277494cf0359b69b441aa
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/282773
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>

src/goLanguageServer.ts

@@ -164,6 +164,12 @@ export async function startLanguageServerWithFallback(ctx: vscode.ExtensionConte
 
 		const started = await startLanguageServer(ctx, cfg);
 
+		if (!started && goConfig['useLanguageServer'] === true) {
+			// We already created various notification - e.g. missing gopls, ...
+			// So, just leave a log message here instead of issuing one more notification.
+			outputChannel.appendLine(
+				`Failed to start the language server (${cfg.serverName}). Falling back to default language providers...`);
+		}
 		// If the server has been disabled, or failed to start,
 		// fall back to the default providers, while making sure not to
 		// re-register any providers.