Add custom error type
Add custom error type

This is useful in case the application using the client api need to get
the response code. A 500 error often has to be handled differently than
a 409 for example.

ehazlett/dockerclient,selvik/dockerclient,abronan/dockerclient,denverdino/dockerclient,BlueDragonX/dockerclient,curoverse/dockerclient,samalba/dockerclient,yhat/go-docker,ibuildthecloud/dockerclient,mountkin/dockerclient,yp-engineering/dockerclient,dnephin/dockerclient,bajh/dockerclient,jimmyxian/dockerclient,donhcd/dockerclient,divideandconquer/dockerclient,appfront/go-docker,flavio/dockerclient,crhym3/dockerclient,turtlebender/dockerclient