Using rabin chunker in browser error swallowed?

[alanshaw]

Error should be being thrown:
https://github.com/ipfs/js-ipfs-unixfs-engine/blob/master/src/chunker/rabin.js#L9-L15

...but attempting to use in browser doesn't appear to cause an error ipfs/js-ipfs#1633

[achingbrain]

I've updated the code to return a pull-through transform stream that errors instead of throwing, should be a bit nicer to work with.

[achingbrain]

N.b. I can't release this yet because interface-datastore has been updated so this instanceof check fails. Once js-ipfs has interface-datastore@0.0.6 it should start working again.