Make exception message more friendly

I've met  a exception such as "not talking to master and retries used up,error from server",but i don't know why.
At last,i found it that because i query data from secondary mongodb not master,so i have to set slaveOk to be true.

But if the exception message contained server error message,i can fixed it much more easily.So i pull this request to add server error in exception message.Thanks a lot.