all: use errgo instead of fmt.Errorf

instance/container.go

 // Copyright 2013 Canonical Ltd.
 // Licensed under the AGPLv3, see LICENCE file for details.
 
 package instance
 
-import (
-        "fmt"
-)
+import "github.com/juju/errgo/errors"
 
 type ContainerType string
 
 const (
         NONE = ContainerType("none")
         LXC  = ContainerType("lxc")
         KVM  = ContainerType("kvm")
 )
 
 // ContainerTypes is used to validate add-machine arguments.
@@ skipping 13 matching lines @@
 }
 
 // ParseContainerType converts the specified string into a supported
 // ContainerType instance or returns an error if the container type is invalid.
 func ParseContainerType(ctype string) (ContainerType, error) {
         for _, supportedType := range ContainerTypes {
                 if ContainerType(ctype) == supportedType {
                         return supportedType, nil
                 }
         }
-        return "", fmt.Errorf("invalid container type %q", ctype)
+        return "", errors.Newf("invalid container type %q", ctype)
 }