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 i think it may be not that good.. the logic is only used when serveKeys, and it is reasonable to trim StoreKeysPrefix in serveKeys. But it is weird to put the logic into the general writeError function. If I change the function name to something more specific, like writeKeyError, it seems weird to use it in other places then..