devserver: Check whether devserver can run before rootfs update.

BUG=chromium:718532
TEST=Test locally: provision a DUT with old version of ChromeOS.

Change-Id: I3a7833ab2f81246015f8e228fd6de16509b93fa3
Reviewed-on: https://chromium-review.googlesource.com/500048
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/devserver.py b/devserver.py
index 5e6cc0f..fec5001 100755
--- a/devserver.py
+++ b/devserver.py
@@ -97,10 +97,14 @@
 # devserver on host.
 try:
   import cros_update
-  import cros_update_progress
 except ImportError as e:
   _Log('cros_update cannot be imported: %r', e)
   cros_update = None
+
+try:
+  import cros_update_progress
+except ImportError as e:
+  _Log('cros_update_progress cannot be imported: %r', e)
   cros_update_progress = None
 
 # only import setup_chromite before chromite import.