Proposal: add ErrorHandler to ServerOptions

[jba]

At various points in the serving infrastructure, an error is produced out-of-band of any particular client RPC or API call. We should add an ErrorHandler to allow users of the SDK to receive these errors.

Open question: should this be scoped to the Server, or to the Session? Probably it should be scoped to the session, in which case this belongs on something like the SessionOptions being discussed in #148.

[findleyr]

@jba I added some description of this issue, and promoted this to a proposal. I'm not sure the API is obvious though.

[jba]

The API is at least

type SessionOptions struct {
// ...
ErrorHandler func(error)
}

but we also might want the same on ServerOptions as a convenience. You're right, it's not obvious. Will think about it.

[findleyr]

Let's just stick this in ServerOptions, to keep things simple.

[maciej-kisiel]

Some input from the community would be helpful to gauge the need for this.