MB-10105 Treat all errors from gen_smtp_client:send as errors.

Change-Id: I3848c065f08b1fe89f0d64eb4a2fbcc4dfe8d7b2
Reviewed-on: http://review.couchbase.org/44432
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>