platform_CrosDisksDBus: Verify error code in unmount test

BUG=chromium:873903
TEST=run platform_CrosDisksDBus

Change-Id: Ia66b7321ecfb818c1311f86263f52b726638ad09
Reviewed-on: https://chromium-review.googlesource.com/1175512
Commit-Ready: Anand Mistry <amistry@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/client/cros/cros_disks.py b/client/cros/cros_disks.py
index abdad77..46a6541 100644
--- a/client/cros/cros_disks.py
+++ b/client/cros/cros_disks.py
@@ -346,10 +346,13 @@
         Args:
             path: The device or mount path to unmount.
             options: A list of options used for unmounting the path.
+
+        Returns:
+            The mount error code.
         """
         if options is None:
             options = []
-        self.interface.Unmount(path, dbus.Array(options, signature='s'))
+        return self.interface.Unmount(path, dbus.Array(options, signature='s'))
 
     def wait_for_mount_completion(self):
         """Waits for the CrosDisks MountCompleted signal.