Catch more errors when trying to check certificates

[mythmon]

It looks like when we check certificates, we need to be a bit more careful about the exceptions that can be thrown by the network.

https://sentry.prod.mozaws.net/operations/normandy-stage/issues/614319/

...snip
  File "normandy/health/api/views.py", line 131, in heartbeat_check_detail
    errors = check(app_configs=None)
  File "normandy/recipes/checks.py", line 73, in recipe_signatures_use_good_certificates
    signing.verify_x5u(url, expire_early)
  File "normandy/recipes/signing.py", line 171, in verify_x5u
    req = requests.get(url)
...snip
SSLError: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:749)