[improvement]: display informational message vs error handler on user initialted startup and shutdown

[dolesec]

FreePBX Version

FreePBX 17

Improvement Description

Today if one is viewing the web interface after initiating a reboot/shutdown or when the system is starting up there is a good chance the error handler page will be displayed. Instead of this, an information message should be displayed. The message should note the system is either shutting down or that the system is still starting.

Switchvox behaves in this manner as well as other open-source projects like TrueNas.

Below is an example of an early page load during FreePBX startup - this is undesirable and should be replaced with a notice that the system is still in start-up; the opposite should occur on shutdown for either a reboot or full system stop.