errors: Improve tests, unify interfaces, wrapping

state/apiserver/usermanager/usermanager.go

 // Copyright 2014 Canonical Ltd.
 // Licensed under the AGPLv3, see LICENCE file for details.
 
 package usermanager
 
 import (
         "fmt"
 
-        "github.com/loggo/loggo"
+        "github.com/juju/loggo"
 
-        "launchpad.net/juju-core/errors"
         "launchpad.net/juju-core/state"
         "launchpad.net/juju-core/state/api/params"
         "launchpad.net/juju-core/state/apiserver/common"
 )
 
 var logger = loggo.GetLogger("juju.state.apiserver.usermanager")
 
 // UserManager defines the methods on the usermanager API end point.
 type UserManager interface {
         AddUser(arg params.EntityPasswords) (params.ErrorResults, error)
@@ skipping 39 matching lines @@
                 result.Results[0].Error = common.ServerError(err)
                 return result, err
         }
         for i, arg := range args.Changes {
                 if !canWrite(arg.Tag) {
                         result.Results[0].Error = common.ServerError(common.ErrPerm)
                         continue
                 }
                 _, err := api.state.AddUser(arg.Tag, arg.Password)
                 if err != nil {
-                        err = errors.NewAlreadyExists(err, "Failed to create user")
+                        err = fmt.Errorf("Failed to create user: %v", err)
                         result.Results[i].Error = common.ServerError(err)
                         continue
                 }
         }
         return result, nil
 }
 
 func (api *UserManagerAPI) RemoveUser(args params.Entities) (params.ErrorResults, error) {
         result := params.ErrorResults{
                 Results: make([]params.ErrorResult, len(args.Entities)),
@@ skipping 16 matching lines @@
                         continue
                 }
                 err = user.Deactivate()
                 if err != nil {
                         result.Results[i].Error = common.ServerError(fmt.Errorf("Failed to remove user: %s", err))
                         continue
                 }
         }
         return result, nil
 }