Throw an error when response is unserializable

[Mrtenz]

Previously, when a Snap would attempt to send an unserializable response, the stream would close with a cryptic error:

Error: PortDuplexStream - disconnected
  at PortDuplexStream._write (common-7.js:32090:23)
  at doWrite (common-8.js:42947:64)

This is because streams cannot serialize non-JSON-compatible values. I've added a check to make sure a response is serializable, and throw an error otherwise. Now, when a Snap tries to send an unserializable response, it will result in a more understandable JSON-RPC error:

{
  "code": -32603,
  "message": "JSON-RPC responses must be JSON serializable objects"
}