Consider using SuppressedError when invoking a callback in error steps

[domfarolino]

The catch() and inspect() operators have internal observers whose error steps invoke a developer-supplied callback with the given error value they receive. If that callback throws an error, then we must invoke Subscriber's error() method with the error thrown from the callback instead of the original error they intended to surface. This clobbering of the original error is exactly what SuppressedError intends to alleviate (first mentioned in #174), so we should consider using it in those operators.