Show how to handle errors in .NET clients

[josephdecock]

When IdentityServer returns protocol errors, the .NET OIDC handler doesn't always handle that gracefully. This is most easily seen from a user clicking the cancel button on the login screen.

Similarly, other errors in the handler can result in the user seeing an error. For example, if the user creates a bookmark in the middle of the flow.

We should show how to catch the error events and do reasonable things.