Fail gracefully when we can't access session data.

When Rails 3 is missing `config.secret_token`, it raises an
ArgumentError when attempting to access the session. This handles that
scenario. Rails 4 raises a RuntimeError before initializing Rails, so
Honeybadger is never invoked (this is better IMO, since important
configuration should be validated before booting the app at all).

Conflicts:
	lib/honeybadger/notice.rb
Be generous when rescuing session exceptions.

Rails raises ActionDispatch::Session::SessionRestoreError when a session
can't be restored/deserialized. Really, there's no situation where we'd
want to re-raise an exception when attempting to locate session data.