Improve error-handling documentation (EDGE-2051)

[DavidMGross]

Common questions are related to error handling such as retry, exponential back off, checked exceptions, user functions throwing, and handling different errors differently.

Let's add a section to the docs with code examples for these things and explaining the Rx contract with idiomatic solutions.

It should show how to convert from imperative try/catch thinking.

See: https://github.com/ReactiveX/RxJava/wiki/Error-Handling which covers some but not all of this

[DavidMGross]

Duplicate of #1544