UI behaviour when controller is unreachable

[cc-a]

Currently if the controller UI is opened and the root controller is unreachable (e.g. because it is not running) then the Finite State Machine and Application Tree boxes on the page simply do not populate and the user receives no feedback. Under the hood the connectivity server raises an exception when it is unable to find the root controller and the htmx requests used to populate the page elements get a 500 response.

A simple improvement we could make here would be to handle the error raised by the connectivity server and return a generic "Unable to contact root controller" message.

We already have some infrastructure to help with this in the form of the main/views/utils.py::handle_errors decorator. This could be fairly straightforwardly enhanced to allow a customisable error message and applied to the relevant view functions in the controller.