make ConfigCoordinator errors more descriptive

Had a problem when any update to config database from mongos resulted in `"could not verify config servers were active and reachable before write."`, yet all config servers were up and running.
After this fix the error message will include config servers responses. Error with message like: 

    could not verify config servers were active and reachable before write. config responses { 
        mongo-cfg1:27010: { ok: 1 }, 
        mongo-cfg2:27010: { ok: 1 }, 
        mongo-cfg3:27010: { ok: 0, code: 15907, errmsg: \"could not initialize sharding on connection mongo-cfg3:27010 (172.20.0.3) :: caused by :: mongos specified a different config database string : stored : mongo:27110,mongo2:27110,mongo-cfg3:27010 vs given : mongo-cfg1:27010,mongo-cfg2:27010,mongo-cfg3:27010\" } 
    }

makes it much easier to identify and fix the problem.