SMTP: Do not ignore TLS errors

This fixes a CVE-2020-15047 (category: CWE-295). Since commit 0083eea5
which added initial, experimental support for SMTP message submission,
we have apparently never implemented proper SSL/TLS error handling, and
the code has ever since just kept silently ignoring any certificate
verification errors.  As a result, Trojita was susceptible to a MITM
attack when sending e-mails. The information leaked include user's
authentication details, including the password, and the content of sent
messages.

Sorry for this :(.

Now, this patch re-enabes proper TLS error handling. It was not possible
to directly re-use our code for TLS key pinning which we are using for
IMAP connections. In the Qt TLS code, the decision to accept or not
accept a TLS connection is a blocking one, so the IMAP code relies upon
the protocol state machine (i.e., another layer) for deciding whether to
use or not to use the just-established TLS connection. Implementing an
equivalent code in the SMTP library would be nice, but this hot-fix has
a priority. As a result, SMTP connections to hosts with, e.g.,
self-signed TLS certs, are no longer possible. Let's hope that this is
not a practical problem with Lets Encrypt anymore.

Thanks to Damian Poddebniak for reporting this bug.

Change-Id: Icd6bbb2b0fb3e45159fc9699ebd07ab84262fe37
CVE: CVE-2020-15047
BUG: 423453