etcdhttp: trim StoreKeysPrefix from error in serveKeys

It returns error messaage like this now:
'{"errorCode":100,"message":"Key not found","cause":"/1/pants","index":10}'

The commit trims '/1' prefix from cause field if exists.

This is a hack to make it display well. It is correct because all error causes
that contain Path puts Path at the head of the string.

fixes #1389 
@jonboulle the error message for `GET /v2/admin/members/` comes from cluster, and doesn't originally come from store, so it is okay.