Add a type for fallible record/follows-from, and error handling wrapper

[hawkw]

See #104:

Currently, the Subscriber::follows_from and Subscriber::record_$TYPE methods return Results. This was done so that the decision of how to handle errors that might occur when calling those methods to be made by the user (at the callsite), rather than by a library implementing Subscribers.

@carllerche suggests that the error handling decision shouldn't be made at the callsite --- as there's not much the call site can do besides either logging the error or panicking, the caller shouldn't be responsible for handling these errors. Instead, he suggests that the handling of these errors should happen between the Subscriber and the DIspatcher. We could consider adding a trait (perhaps in the tokio-trace-subscriber crate) for fallible Subscribers, and a type that wraps a fallible subscriber with customizable error-handling behaviour. That wrapper type could implement the core Subscriber infallibly.

The first part (changes to -core) was done in #105. Eventually, we should add the fallible type in the subscriber crate, if there's demand for it.