If I understand, you are looking for a tool/plugin that warns users (via email) when Redmine is down or failing, right?
For this purpose, the tool shared above is adequate. If Redmine is down, you can not notify the users using Redmine (or a plugin/extension), because Redmine itself is down, so you need an external tool that monitor services related to Redmine to determine when Redmine is down and notify the users. You can take a look at the previous link to try to configure this tool.
Note, that this Community forum is intended to solve problems related to the Bitnami Stacks, Virtual Machine or Cloud Images. For information regarding the application itself or customization of the content within the application, we highly recommend checking forums and user guides made available by the project behind the application.
Carlos R. Hernández