message: fix(server.ts): display server startup errors
diff: import * as child from 'child_process';
  import * as carrier from 'carrier';
  +import * as vscode from 'vscode'
  const defaultServerOptions = [
  "--server",
  @@ -142,6 +143,13 @@ class Server {
  console.log(`stderr: ${data}`);
  });
  + this.process.on('error', (e) => {
  + vscode.window.showErrorMessage(
  + "Unable to start the Lean server process: " + e);
  + vscode.window.showWarningMessage(
  + "The lean.executablePath may be incorrect, make sure it is a valid Lean executable");
  + });
  +
  this.process.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
  // TODO(jroesch): status change here // this.onStatusChange()
  