Small fixes in Dawn generator

- Add helpful assert messages in a few places
- Fix a template in the api_struct_info.json for Emscripten

Bug: none
Change-Id: Iee7a4f6ce5947e4f59c4f5a3d9cb5b475758eeea
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/147181
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
diff --git a/generator/dawn_json_generator.py b/generator/dawn_json_generator.py
index e33e73b..a877e91 100644
--- a/generator/dawn_json_generator.py
+++ b/generator/dawn_json_generator.py
@@ -572,7 +572,8 @@
             is_void = method.return_type.name.canonical_case() == 'void'
             if not (is_object or is_void):
                 assert command_suffix in (
-                    wire_json['special items']['client_handwritten_commands'])
+                    wire_json['special items']['client_handwritten_commands']
+                ), command_suffix
                 continue
 
             if command_suffix in (