Also remove operator bool in Error. The Error should be removed
later because it is not necessary for Paddle. We are now using Enforce
to handle error.

* using etcd as fault tolerant training

* update

* workable version, ft not tested

* small fix

* update

* remove TODO

Fix #2567

When using system python, "pip install" needs root previlege. Using virtualenv to avoid this.

cost is Argument, but argument does not have to be cost.