Log error from unmountVolumes on cleanup

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

pkg/system/syscall_unix.go

@@ -6,6 +6,6 @@ import "syscall"
 
 // Unmount is a platform-specific helper function to call
 // the unmount syscall.
-func Unmount(dest string) {
-	syscall.Unmount(dest, 0)
+func Unmount(dest string) error {
+	return syscall.Unmount(dest, 0)
 }

pkg/system/syscall_windows.go

@@ -2,5 +2,6 @@ package system
 
 // Unmount is a platform-specific helper function to call
 // the unmount syscall. Not supported on Windows
-func Unmount(dest string) {
+func Unmount(dest string) error {
+	return nil
 }