vm: output the error when VM failed to start

BUG=None
TEST=None

Change-Id: I0c877d0138bebe3ac925238c5f020db923db648f
Reviewed-on: https://chromium-review.googlesource.com/187916
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
(cherry picked from commit d269cff9f28451dc80c87673f3ae0ec3bc7c5739)
Reviewed-on: https://chromium-review.googlesource.com/188168
diff --git a/lib/vm.py b/lib/vm.py
index c121e6a..cd7c351 100644
--- a/lib/vm.py
+++ b/lib/vm.py
@@ -68,11 +68,11 @@
   if board:
     cmd.extend(['--board', board])
 
-  msg = 'Failed to create the VM image.'
+  msg = 'Failed to create the VM image'
   try:
     cros_build_lib.RunCommand(cmd, enter_chroot=True, cwd=constants.SOURCE_ROOT)
-  except:
-    logging.error(msg)
+  except cros_build_lib.RunCommandError as e:
+    logging.error('%s: %s', msg, str(e))
     raise VMCreationError(msg)
 
   if not os.path.exists(vm_image_path):