(maint) Refactor HTTPS verification tests into a shared examples

This commit refactors the spec tests that ensure puppet displays
meaningful error messages when SSL verification fails, e.g. when the
server certificate doesn't match the hostname we connected to.
(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.
Merge pull request #2954 from joshcooper/ticket/master/PUP-744-rescue-openssl

(PUP-744) Expand scope of OpenSSL rescue block