Switch HMR connection to register bundle entry points via a message

Summary:
This diff builds on the previous ones and changes the setup process from using the WebSocket URL to using a message that is sent after the connection is established. It also exposes a function on the HMRClient that allows registering more bundles, which I will make use of in the next (and hopefully final :D ) diff.

I was initially planning on using structured data, like `{bundleName, platform}` but decided to keep using URLs as that is the format used throughout Metro. In fact, when we parse the options from the URL, we need to re-encode the input URL to create the `sourceMapUrl`. I thought it doesn't make sense to write more code to send structured data over the connection only to re-construct a URL on the server manually.

Finally, I also slightly modified the "Internal Bundler" error that is shown in a RedBox (now used by the websocket connection if an invalid message is received). I removed the "internal" wording from the message and I'm actually attaching the failure message to the error instead of directing users to the Terminal.

Reviewed By: gaearon

Differential Revision: D16162729

fbshipit-source-id: 977fde5f6c2f1c14efb4fd99ed30a6bf95a3b13e