Exception handling for controllers using ActionController::Live

Any exceptions that occured at the view or controller level for a
controller using ActionController::Live would cause the server to either
hang with an open socket indefinitely, or immediately crash (depending
on whether the server was launched with rails s or directly). Changed
the behavior of exceptions to act the same as streaming templates for
html requests, and allow for an on_error callback if needed.
Merge pull request #9604 from sgrif/live_streaming_exceptions

Exceptions raised when using ActionController::Live cause server crash