Ember raise end of stream errors wherever appropriate

[RaasAhsan]

Changes:

1. Simplified fixed-length body parsing logic
2. Fixed several bugs where a body stream silently ended if the peer closed its end of the socket without finishing writing. This raises an error now

[rossabaker]

Looks reasonable, but let's see what @ChristopherDavenport says.

[rossabaker]

There's a standard IOException for this, but since EmberException is a RuntimeException, we can't extend it.