change: return error message directly

[YuanSheng Wang]

We should use the same style between different models.

lua-resty-etcd/lib/resty/etcd/v2.lua

Lines 185 to 190 in 4c80dd1

 if err then 
     if self.is_cluster then 
         err = 'fail request to: ' .. uri .. ' err:' .. tostring(err) 
     end 
     return nil, err 
 end