Fixed typo

src/server/vms/vms.go

@@ -509,7 +509,7 @@ func (vms *VMs)ImportFilesToVM(vm *VM, tarGzFile, vmDir string) error {
 
     // Marks VM as "busy", meaning its disk file is being accessed for a possibly long time.
     if vm.DiskBusy {
-        return nil, errors.New("Cannot import files into VM: disk is already busy")
+        return errors.New("Cannot import files into VM: disk is already busy")
     }
     vm.DiskBusy = true