[Do not merge, RFC]Add dataflow!

With futures, you can call ```value!``` if you want exceptions to bubble up. It'd be rather nice if you could do the same thing with dataflow.

This is the simplest possible solution, if you collectively think this is a worthwhile feature I'll polish it up, add tests, etc.
tests for raising errors
remove redundant test here
cleanup
De-duplicate code
Merge pull request #131 from mastfish/patch-1

Add dataflow! to match value!