Add unexpected files to presubmit error

This makes it easier to debug which files are incorrectly bundled
with other changes.

R=jacktfranklin@chromium.org

Bug: none
Change-Id: Ifdf56da22a164cc2bbc762f79d80f70122f6057e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2817078
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 599cfd6..b75211c 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -97,9 +97,16 @@
         if '.gitignore' in affected_files:
             num_in_dir = num_in_dir + 1
         if num_in_dir < num_affected:
-            results.append(output_api
-                .PresubmitError(('CLs that affect files in "%s" should be limited to these files/directories.' % dir_list) +
-                                ' You can disable this check by adding DISABLE_THIRD_PARTY_CHECK=<reason> to your commit message'))
+            unexpected_files = [
+                file for file in affected_files if file not in affected_in_dir
+            ]
+            results.append(
+                output_api.PresubmitError(
+                    ('CLs that affect files in "%s" should be limited to these files/directories.'
+                     % dir_list) +
+                    ('\nUnexpected files: %s.' % unexpected_files) +
+                    '\nYou can disable this check by adding DISABLE_THIRD_PARTY_CHECK=<reason> to your commit message'
+                ))
             break
 
     return results