[feature] ability to report custom statuses/errors via agent

[mips171]

Just an idea, it might be useful to have the ability to register and report custom errors or statuses via the agent that go beyond the default UNKNOWN, OK, PROBLEM, CRITICAL set shown in the dashboard. For example, user can run any script that can set the custom status, like MOVING.

This could also be useful for users who might be using or evaluating OpenWISP for more than just monitoring the network device, they could use this feature to monitor attached IoT devices, since any program would be able to set the status. Thoughts?