[fidl] More informative error message for library namespace verification.

Change-Id: I76a4b31e38a914313afc39b872c7814f6a25ef11

style/verify-fidl-libraries.py

diff --git a/style/verify-fidl-libraries.py b/style/verify-fidl-libraries.py
index b847f07..88d263b 100755
--- a/style/verify-fidl-libraries.py
+++ b/style/verify-fidl-libraries.py
@@ -58,7 +58,9 @@
                 continue
             namespace = result.group(1)
             if namespace not in args.namespaces:
-                print('Invalid namespace %s (%s)' % (namespace, file))
+                print(
+                    'Invalid namespace %s (%s), namespace must begin with one of [%s].'
+                    % (namespace, file, ', '.join(NAMESPACES)))
                 has_errors = True
 
     return 1 if has_errors else 0