Fix several btrfs driver error messages

Several of the error messages were wrong, but not one of them was helpful, so this fixes them to include the important information (the actual error), and a correct description of which operation failed.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

graphdriver/btrfs/btrfs.go

@@ -99,7 +99,7 @@ func subvolCreate(path, name string) error {
 	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(dir), C.BTRFS_IOC_SUBVOL_CREATE,
 		uintptr(unsafe.Pointer(&args)))
 	if errno != 0 {
-		return fmt.Errorf("Can't create subvolume")
+		return fmt.Errorf("Failed to create btrfs subvolume: %v", errno.Error())
 	}
 	return nil
 }
@@ -126,7 +126,7 @@ func subvolSnapshot(src, dest, name string) error {
 	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(destDir), C.BTRFS_IOC_SNAP_CREATE_V2,
 		uintptr(unsafe.Pointer(&args)))
 	if errno != 0 {
-		return fmt.Errorf("Can't create subvolume")
+		return fmt.Errorf("Failed to create btrfs snapshot: %v", errno.Error())
 	}
 	return nil
 }
@@ -146,7 +146,7 @@ func subvolDelete(path, name string) error {
 	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, getDirFd(dir), C.BTRFS_IOC_SNAP_DESTROY,
 		uintptr(unsafe.Pointer(&args)))
 	if errno != 0 {
-		return fmt.Errorf("Can't create subvolume")
+		return fmt.Errorf("Failed to destroy btrfs snapshot: %v", errno.Error())
 	}
 	return nil
 }