oo-diagnostics test_broker_certificate fixes

Drop a "require 'socket'" statement in the test_broker_certificate check.

This require statement was obviated in commit 9b72939f28a6d35354db74cc7464d879bbd3a81e.


Catch and report if curl returns an error in the test_broker_certificate check.

Previously, test_broker_certificate would ignore curl's return code, and so errors connecting to the broker would turn up as backtraces when test_broker_certificate would try to parse the output from curl.

Let me know if we should do more error checking, for example rescuing from exceptions when parsing the response from the broker.  It is probably a more common problem that the broker fails to start, so it is nice to identify the problem quickly.  It is probably a less common problem that curl succeeds but the response cannot be parsed, so it may not be worth rescuing from such errors to return a nicer error message.