firmware_test: fix error message

When a test image is not found in the USB disk, the error message is
incorrect and misleads people. Let's fix this.

BUG=chrome-os-partner:24835
TEST=None

Change-Id: Ic49ae28ea3bfbd044de7fae226984fd0c59285be
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/201150
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
diff --git a/server/cros/faft/firmware_test.py b/server/cros/faft/firmware_test.py
index 449a612..ed17975 100644
--- a/server/cros/faft/firmware_test.py
+++ b/server/cros/faft/firmware_test.py
@@ -395,7 +395,7 @@
         if not check_passed:
             raise error.TestError(
                 'No Chrome OS %s found on the USB flash plugged into servo' %
-                'install shim' if install_shim else 'test')
+                ('install shim' if install_shim else 'test'))
 
         self.mark_setup_done('usb_check')