Add ability to display status message to user

Add ability to show custom message to users of Fusion to indicate that there is an issue with Nexus. Would be useful when we know that GPFS is going to be down, for example.

No files, tests, or entry points are named. Start by locating Fusion's user-facing status handling and the Nexus outage path, including the GPFS-down example; define how an administrator supplies a custom message, where users see it, and how the message is cleared before implementation.

Written by the indexing model from the issue text.