Bug 1723838 - Tweak error reported from WebExtensions GenerateWebIDLBindings.py on unexpected type for optional parameters. r=willdurand

Differential Revision: https://phabricator.services.mozilla.com/D167591

toolkit/components/extensions/webidl-api/GenerateWebIDLBindings.py

@@ -690,9 +690,10 @@ class WebIDLHelpers:
                 # in this method.
                 continue
 
+            api_path = api_fun.api_path_string
+            pname = param["name"]
             ptype = cls.webidl_type_from_mapping(
-                param,
-                "%s method parameter %s" % (api_fun.api_path_string, param["name"]),
+                param, f"{api_path} method parameter {pname}"
             )
 
             if (
@@ -702,12 +703,12 @@ class WebIDLHelpers:
             ):
                 if ptype != "Function":
                     raise TypeError(
-                        "unexpected optional type. "
-                        "Only Function is expected to be marked as optional"
+                        f"unexpected optional type: '{ptype}'. "
+                        f"Only Function is expected to be marked as optional: '{api_path}' parameter '{pname}'"
                     )
-                ptype = "optional %s" % ptype
+                ptype = f"optional {ptype}"
 
-            params.append("%s %s" % (ptype, param["name"]))
+            params.append(f"{ptype} {pname}")
 
         if api_fun.is_async(schema_group):
             # Add the chrome-compatible callback as an additional optional parameter
@@ -718,7 +719,7 @@ class WebIDLHelpers:
             # of the schema data for the callback parameter and throws if the expected
             # parameter is missing).
             params.append(
-                "optional Function %s" % api_fun.get_async_callback_name(schema_group)
+                f"optional Function {api_fun.get_async_callback_name(schema_group)}"
             )
 
         return params