Add CRITICAL event handler.

[hjoliver]

(separated from #2298)

[hjoliver]

From the original issue #2298 (comment)

@matthewrmshin wrote:

If we have configured a failed handler and a critical handler, I guess both will run?

Yes, as implemented, critical handlers get all CRITICAL events. This seems logical to me, but obviously failed handlers will get many/most of the same events. This isn't necessarily a bad thing as critical and failed handlers could do different things. Alternatively we could pass only non-failure CRITICAL events to critical handlers. Or allow both options?

[matthewrmshin]

Let's go with the simple implementation unless people scream.

[hjoliver]

Ah, which is "the simple implementation" in your opinion? (as it is now?)

[matthewrmshin]

As implemented is good enough unless people scream.

[hjoliver]

OK, glad you agree 😁 I'll just add a test...

[hjoliver]

(actually, I already added this to the existing test).

[hjoliver]

@matthewrmshin - please assign a second reviewer.