* agent: return a non-zero exit code on error

* agent/template: always return on template server error, add case for error_on_missing_key

* agent: fix tests by updating Run params to use an errCh

* agent/template: add permission denied test case, clean up test var

* agent: use unbuffered errCh, emit fatal errors directly to the UI output

* agent: use oklog's run.Group to schedule subsystem runners (#9761)

* agent: use oklog's run.Group to schedule subsystem runners

* agent: clean up unused DoneCh, clean up agent's main Run func

* agent/template: use ts.stopped.CAS to atomically swap value

* fix tests

* fix tests

* agent/template: add timeout on TestRunServer

* agent: output error via logs and return a generic error on non-zero exit

* fix TestAgent_ExitAfterAuth

* agent/template: do not restart ct runner on new incoming token if exit_after_auth is set to true

* agent: drain ah.OutputCh after sink exits to avoid blocking on the channel

* use context.WithTimeout, expand comments around ordering of defer cancel()