better error handling; data channel fixes; improved sample chat2 app 

* Added error handling for uproxypeerconnection
* Fixes to the chat2 demo to work correctly with sending on multiple channels
* Fixes to data channel's sending of strings (empty string now handled correctly)
* Sourcemaps included for third_party js code 
A comment would be good here. A one liner is fine, e.g.:
```js
// Type for for core provider functions' continuation argument.
```