Add ErrPrerequisites to improve misleading errors

There are two cases where we can't use a graphdriver:

1) the graphdriver itself isn't supported by the system
2) the graphdriver is supported by some configuration/prerequisites are
missing

This introduces a new error for the 2) case and uses it when trying to
run docker with btrfs backend on a non-btrfs filesystem.

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)

daemon/graphdriver/btrfs/btrfs.go

@@ -18,6 +18,10 @@ import (
 	"unsafe"
 )
 
+const (
+	btrfsSuperMagic = 0x9123683E
+)
+
 func init() {
 	graphdriver.Register("btrfs", Init)
 }
@@ -30,8 +34,8 @@ func Init(home string) (graphdriver.Driver, error) {
 		return nil, err
 	}
 
-	if buf.Type != 0x9123683E {
-		return nil, graphdriver.ErrNotSupported
+	if buf.Type != btrfsSuperMagic {
+		return nil, graphdriver.ErrPrerequisites
 	}
 
 	return &Driver{

daemon/graphdriver/driver.go

@@ -44,7 +44,8 @@ var (
 		"vfs",
 	}
 
-	ErrNotSupported = errors.New("driver not supported")
+	ErrNotSupported  = errors.New("driver not supported")
+	ErrPrerequisites = errors.New("Prerequisites for driver not satisfied (wrong filesystem?)")
 )
 
 func init() {
@@ -78,7 +79,7 @@ func New(root string) (driver Driver, err error) {
 	for _, name := range priority {
 		driver, err = GetDriver(name, root)
 		if err != nil {
-			if err == ErrNotSupported {
+			if err == ErrNotSupported || err == ErrPrerequisites {
 				continue
 			}
 			return nil, err
@@ -89,7 +90,7 @@ func New(root string) (driver Driver, err error) {
 	// Check all registered drivers if no priority driver is found
 	for _, initFunc := range drivers {
 		if driver, err = initFunc(root); err != nil {
-			if err == ErrNotSupported {
+			if err == ErrNotSupported || err == ErrPrerequisites {
 				continue
 			}
 			return nil, err

daemon/graphdriver/graphtest/graphtest.go

@@ -31,7 +31,7 @@ func newDriver(t *testing.T, name string) *Driver {
 
 	d, err := graphdriver.GetDriver(name, root)
 	if err != nil {
-		if err == graphdriver.ErrNotSupported {
+		if err == graphdriver.ErrNotSupported || err == graphdriver.ErrPrerequisites {
 			t.Skip("Driver %s not supported", name)
 		}
 		t.Fatal(err)