(PUP-744) Expand scope of OpenSSL rescue block

The persistent http connection work introduced a regression, preventing the agent from displaying useful error messages when SSL verification fails, e.g. the server's SSL certificate doesn't match the hostname the agent tried to connect to. The connection_spec test didn't catch the issue, because those tests execute with the non-caching pool, which always uses non-persistent connections.

The root cause is because the Connection class assumed http connections are started by ruby in the `Net::HTTP#request` method, so the OpenSSL rescue block wrapped that call.

However, in order to use persistent http connection, the caller needs to explicitly start the connection prior to calling Net::HTTP#request, which happens in the outer `Connection#with_connection` method.

This commit expands the scope of the rescue block. This way we receive meaningful error messages if the connection is started explicitly for persistent connections, or on-demand for non-persistent connections. It also executes the ssl verification tests using persistent connections.

Also note that `with_connection` is private, so the fact that `Pool#with_connection` or `Net::HTTP#request` can start the connection is not visible to users of the Connection class.

Verified that the `external_ca_support/apache_external_root_ca.rb` and `helpful_error_message_when_hostname_not_match_server_certificate.rb` acceptance tests pass on rhel6