Update checkdeps trailing slash failure message

Update the failure message to indicate that the trailing slash should be
removed.

Bug: 1084817
Change-Id: Id4ed3533531b45c1762d66068700f93fee6db9a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2650169
Auto-Submit: Theresa  <twellington@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#847247}

buildtools/checkdeps/rules.py

@@ -79,8 +79,8 @@ def ParseRuleString(rule_string, source):
   # Ban them.
   if rule_string[-1] == '/':
     raise Exception(
-      'The rule string "%s" ends with a "/" which is not allowed' %
-      rule_string)
+      'The rule string "%s" ends with a "/" which is not allowed.'
+      ' Please remove the trailing "/".' % rule_string)
 
   return rule_string[0], rule_string[1:]