Add a "missing semicolon" hint to "no definitions found" error

It took me a while to figure out what was wrong with my valid-looking
new IDL file, this diagnostic would have saved me some time. However,
I don't know if it's potentially confusing for other errors people
might make, or if there are better ways to address this.

Bug: 1041101
Change-Id: Ifae76b7d77e558f1888be0b70ae52fd53fc7514a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995712Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733039}

third_party/blink/renderer/bindings/scripts/idl_reader.py

@@ -68,8 +68,7 @@ def validate_blink_idl_definitions(idl_filename, idl_file_basename,
             len(definitions.enumerations) + len(definitions.typedefs) +
             len(definitions.callback_functions))
         if number_of_definitions == 0:
-            raise Exception(
-                'No definition found in %s' % idl_filename)
+            raise Exception('No definition found in %s. (Missing semicolon?)' % idl_filename)
         return
     target = targets[0]
     if target.is_partial: