fixed dialyzer warning

It looks like lhttpc:request defines error case as {error,
atom()}. While we know that in case of ssl issues it may return
{error, {{tls_error, _}, _}}. Instead of fixing lhttpc I decided to
trick dialyzer with use of nif_error.

Change-Id: Ib8065fec46faacf16828011e28a44271b793993b
Reviewed-on: http://review.couchbase.org/40270
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>