avformat/http: pass return code from http_open_cnx_internal() on its failure

Previously, AVERROR(EIO) was returned on failure of
http_open_cnx_internal(). Now the value is passed to upper level, thus
it is possible to distinguish ECONNREFUSED, ETIMEDOUT, ENETUNREACH etc.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>