Return HTTP 502 from failed phabricator heartbeat checks

[mars-f]

To test this change, add the following to docker-compose.override.yml:

version: '2'
services:
  lando-api:
    environment:
      - PHABRICATOR_URL=https://nosuchhost.test

and hit http://:8888/heartbeat.

The container log should output a WARNING level message. It will not output INFO or DEBUG messages because we don't have the loggers configured for that yet (we should implement mozlogging instead).

[purelogiq]

lgtm!

[ckolos]

I have some concern about monitoring the health of phabricator from this application because phabricator being down doesn't result in a hard-down condition for lando. Whenever we do a phabricator deploy that will involve downtime (should only be schema changes), we will need to expect to receive an alert from lando reflecting that. I'm r+ on this right now with the understanding that we may wish to remove this in the future.