If a user hits this error it would be helpful to know tagstore name.

There are several bug reports on this error happening, and error is
not helpful unless you read the code.  Google brings up removing
the repositories.btrfs file.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

daemon/daemon.go

@@ -686,7 +686,7 @@ func NewDaemon(config *Config, registryService *registry.Service) (daemon *Daemo
 	}
 	repositories, err := graph.NewTagStore(filepath.Join(config.Root, "repositories-"+d.driver.String()), tagCfg)
 	if err != nil {
-		return nil, fmt.Errorf("Couldn't create Tag store: %s", err)
+		return nil, fmt.Errorf("Couldn't create Tag store %s: %s", "repositories-"+d.driver.String(), err)
 	}
 
 	d.netController, err = initNetworkController(config)