When websocket disconnects during `browserType.connect()` call, the
error looks like this now:
```
browserType.connect: Custom error message received over WebSocket
```
Previously, it was a generic error:
```
browserType.connect: Target page, context or browser has been closed
```
Instead of having `didClose` based on page creation/destruction and
`didDisconnect` based on session lifetime, we make session lifetime
being managed by the `CRPage`/`FFPage`/`WKPage` instead.
Potential fixes to avoid startup races:
- Wait for "juggler listening" message.
- Make sure `transport.onclose` is called when connecting to the transport after the actual pipe closure.
