chromite: Incorporate some readability comments.

This CL fixes some readability problems from reviewing my body of code.

BUG=none
TEST=unittest

Change-Id: I1b703f03475c20425204ab8af366ca1207f844fe
Reviewed-on: https://chromium-review.googlesource.com/209633
Reviewed-by: Nam Nguyen <namnguyen@chromium.org>
Commit-Queue: Nam Nguyen <namnguyen@chromium.org>
Tested-by: Nam Nguyen <namnguyen@chromium.org>
diff --git a/scripts/test_image.py b/scripts/test_image.py
index c47084c..f97a7c5 100755
--- a/scripts/test_image.py
+++ b/scripts/test_image.py
@@ -47,7 +47,7 @@
     # Assume base image.
     image_file = os.path.join(image_path, constants.BASE_IMAGE_NAME + '.bin')
     if not os.path.exists(image_file):
-      raise ValueError('%s does not contain base image' % image_path)
+      raise ValueError('Cannot find base image %s' % image_file)
   elif os.path.isfile(image_path):
     image_file = image_path
   else: