Add payload to EtcdException and more elabrated Etcd exceptions

The following exception were added:
EtcdKeyError(EtcdException) - Generic Etcd key error
EtcdKeyNotFound(EtcdKeyError) - Etcd key not found (100)
EtcdNotFile(EtcdKeyError) - Etcd not a file (102)
EtcdNotDir(EtcdKeyError) - Etcd not a directory (104)
EtcdAlreadyExist(EtcdKeyError) - Etcd already exist (105)
EtcdEventIndexCleared(EtcdException) - Etcd event index is outdated and cleared (401)