Make errors from graphdriver init friendlier

In several cases graphdriver were just returning the low-level syscall
error and that was making it all the way up to the daemon logs and in
many cases was difficult to tell it was even coming from the graphdriver
at all.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/daemon.go

@@ -876,7 +876,7 @@ func NewDaemonFromDirectory(config *Config, eng *engine.Engine) (*Daemon, error)
 	// Load storage driver
 	driver, err := graphdriver.New(config.Root, config.GraphOptions)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("error intializing graphdriver: %v", err)
 	}
 	log.Debugf("Using graph driver %s", driver)
 

daemon/graphdriver/aufs/aufs.go

@@ -378,7 +378,7 @@ func (a *Driver) mount(id, mountLabel string) error {
 	}
 
 	if err := a.aufsMount(layers, rw, target, mountLabel); err != nil {
-		return err
+		return fmt.Errorf("error creating aufs mount to %s: %v", target, err)
 	}
 	return nil
 }

daemon/graphdriver/overlay/overlay.go

@@ -301,7 +301,7 @@ func (d *Driver) Get(id string, mountLabel string) (string, error) {
 
 	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lowerDir, upperDir, workDir)
 	if err := syscall.Mount("overlay", mergedDir, "overlay", 0, label.FormatMountLabel(opts, mountLabel)); err != nil {
-		return "", err
+		return "", fmt.Errorf("error creating overlay mount to %s: %v", mergedDir, err)
 	}
 	mount.path = mergedDir
 	mount.mounted = true