Mojo cpp bindings: report the reason of validation failure.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
@@ skipping 255 matching lines @@
         'public/cpp/bindings/lib/message_queue.cc',
         'public/cpp/bindings/lib/message_queue.h',
         'public/cpp/bindings/lib/no_interface.cc',
         'public/cpp/bindings/lib/router.cc',
         'public/cpp/bindings/lib/router.h',
         'public/cpp/bindings/lib/shared_data.h',
         'public/cpp/bindings/lib/shared_ptr.h',
         'public/cpp/bindings/lib/string_serialization.h',
         'public/cpp/bindings/lib/string_serialization.cc',
         'public/cpp/bindings/lib/sync_dispatcher.cc',
+        'public/cpp/bindings/lib/validation_errors.cc',
+        'public/cpp/bindings/lib/validation_errors.h',
       ],
     },
     {
       'target_name': 'mojo_js_bindings',
       'type': 'static_library',
       'include_dirs': [
         '..'
       ],
       'sources': [
         'public/js/bindings/constants.cc',
@@ skipping 140 matching lines @@
           },
           'dependencies': [
             'mojo_public_java',
           ],
           'includes': [ '../build/java.gypi' ],
         },
       ],
     }],
   ],
 }