[MHD-128] reject cases that are strictly invalid

[theunkn0wn1]

This PR resolves MHD-128.

Specifically, mecha will now reject any incoming announcement that is strictly invalid, ie the system, client, or platform are null.

[shatteredbeam]

Id change the output string to something more descriptive.

Something like 'Invalid Announcement, missing (Case Number|System|Platform)'

Otherwise its just going to confuse people not familiar with the process.

[theunkn0wn1]

Given this is a fail-safe measure, and only is triggered when some other component glares at announcer goes wrong, all end users really need to know is something went wrong (strictly invalid) and that the dispatch needs to manually inject the case.

I feel specifying exactly what went wrong would only make the message longer and thus clutter the channel.

[kenneaal]

Given that the announcer only forwards what the client plugin tells it, you can turn that glare of yours elsewhere. :P