Global error callback

[justincy]

Now that response middleware isn't called for request errors, we need a method for registering a global error callback; i.e. a callback that gets called whenever there's a request error (only network errors, not HTTP errors). Response callbacks would still be called. I can think of two use cases this fulfills:

1. Logging
2. If your app, client side, wants to show an error toast whenever there's been a problem instead of each component of the app doing it's own error handling (like many web apps do when your connection is flaky and it's trying to reconnect).