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.
Added documentation to the FormBuilder class, should help
clarify issue #10115. Also made the field_helpers an explicit list of
methods.
Merge pull request #10122 from wangjohn/adding_documentation_to_form_builder

Added documentation to the FormBuilder class

Closes #10115
Merge pull request #9604 from sgrif/live_streaming_exceptions

Exceptions raised when using ActionController::Live cause server crash