Throwing errors should not always crash the Snap

[Mrtenz]

Currently, when you throw an error in the onRpcRequest handler for example, the entire Snap will crash.

export const onRpcRequest = (...) => {
  // ...
  throw new Error('Invalid parameters.');
};

This is problematic when there are multiple requests happening at the same time, because all requests will be canceled.

We should add a way for a Snap to throw an error while not causing the entire Snap to crash, but rather respond with an error to the specific RPC request.

[Mrtenz]

#1841.