Hook up txacme's "panic" callable to do something more visible

[JayH5]

http://txacme.readthedocs.io/en/latest/using.html#issuing-service

panic (Callable[[Failure, str], Deferred]) – A callable invoked with the failure and server name when reissuing fails for a certificate expiring in the panic_interval. For example, you could generate a monitoring alert. The default callback logs a message at CRITICAL level.

Logging as CRITICAL may not be the most visible thing to do if marathon-acme is just quietly running in a container somewhere... Send an email? Sentry error?