more robust startup;

--- a/src/Tools/VSCode/extension/src/extension.ts	Mon Jan 02 14:44:14 2017 +0100
+++ b/src/Tools/VSCode/extension/src/extension.ts	Mon Jan 02 15:00:55 2017 +0100
@@ -12,23 +12,26 @@
   let isabelle_home = vscode.workspace.getConfiguration("isabelle").get<string>("home");
   let isabelle_args = vscode.workspace.getConfiguration("isabelle").get<Array<string>>("args");
 
-  let run = {
-    command: path.join(isabelle_home, "bin", "isabelle"),
-    args: ["vscode_server"].concat(isabelle_args)
-  };
-  let server_options: ServerOptions =
-  {
-    run: run,
-    debug: {
-      command: run.command,
-      args: run.args.concat(["-L", path.join(context.extensionPath, "protocol.log")]) }
-  };
-  let client_options: LanguageClientOptions = {
-    documentSelector: ["isabelle", "isabelle-ml"]
-  };
+  if (isabelle_home == "") vscode.window.showErrorMessage("Missing isabelle.home settings")
+  else {
+    let run = {
+      command: path.join(isabelle_home, "bin", "isabelle"),
+      args: ["vscode_server"].concat(isabelle_args)
+    };
+    let server_options: ServerOptions =
+    {
+      run: run,
+      debug: {
+        command: run.command,
+        args: run.args.concat(["-L", path.join(context.extensionPath, "protocol.log")]) }
+    };
+    let client_options: LanguageClientOptions = {
+      documentSelector: ["isabelle", "isabelle-ml"]
+    };
 
-  let disposable = new LanguageClient("Isabelle", server_options, client_options, false).start();
-  context.subscriptions.push(disposable);
+    let disposable = new LanguageClient("Isabelle", server_options, client_options, false).start();
+    context.subscriptions.push(disposable);
+    }
 }
 
 export function deactivate() { }