Throw if no onError handler specified

[benjchristensen]

If an onError handler is not provided we should throw an exception if an error occurs even though it will likely show up on some other thread.

[mttkay]

To back this up, the relevant section from the Rx Design Guildelines says:

"E.g. when calling the Subscribe method that only has an onNext argument, the OnError behavior will be to rethrow the exception on the thread that the message comes out from the observable sequence. The OnCompleted behavior in this case is to do nothing."

(v1.0, section 5.2, p11)

[mttkay]

Is this closed with the above PR?

[mttkay]

I noticed this went into 0.9.1 👍