Polish for the Environment setting

While updating the documentation for the Environment setting, I found a few issues that could be polished:

• If you set the Environment variable via `civicrm.settings.php`, outbound mail is not disabled.  This is going to give a nasty surprise to someone.  The guts of the `onChangeEnvironmentSetting` method should be extracted, and called when the Outbound Email page is loaded and before mail is sent.
• Only `job.process_mailing` returns an error when the Environment is set to non-production.  All scheduled jobs should return an error.