When launched with --extension or --profile, launchPersistentContext()
is used which sets isPersistentContext=true but leaves this.browser as
null. The guard in newTab() checked !this.browser, causing a false
"Browser not launched" error even though the browser was running.
Replace !this.browser with !this.isLaunched(), which already accounts
for both launch paths (browser !== null || isPersistentContext).
Also improve the error message in newWindow() to clarify that it is
not supported in persistent context mode, since it requires a Browser
object to create a new context.
Fixes#411
