Bug: Calling _blockAndLogStreamer method when it's undefined

[fabioberger]

In 0x Portal, I intermittently receive the following error originating from 0x.js:

Cannot read property 'unsubscribeFromOnLogAdded' of undefined

From this line:
https://github.com/0xProject/0x.js/blob/development/packages/0x.js/src/contract_wrappers/contract_wrapper.ts#L168

Inspecting the code, it seems that _blockAndLogStreamer should actually be called _blockAndLogStreamerIfExists since it can be undefined. Further, we should check whether it exists before calling methods on it.

It would also be good to figure out why _stopBlockAndLogStream is being called multiple times.

[fabioberger]

Fixed in #262