SSL: logging level of "peer closed connection in SSL handshake".

Previously, the NGX_LOG_INFO level was used unconditionally.  This is
correct for client SSL connections, but too low for connections to
upstream servers.  To resolve this, ngx_connection_error() now used
to log this error, it will select logging level appropriately.

With this change, if an upstream connection is closed during SSL
handshake, it is now properly logged at "error" level.