Fixes #3342

Two bugs were fixed:

* Closing the StreamFramer's exitCh before setting the error means other
  goroutines blocked on exitCh closing could see the error as nil. This
  was *not* observered.
* parseFramerError on Windows would fall through and return an
  improperly captured nil err variable. There's no need for
  parseFramerError to be a closure which fixes the confusion.

I hate adding "this should never happen" checks, but causing a tight
loop that OOMs Nomad is just too easy in this code otherwise.