Fix error message when `app_service_config_files` validation fails (#15614)

The second argument of `ConfigError` is a path, passed as an optional
`Iterable[str]` and not a `str`. If a string is passed directly,
Synapse unhelpfully emits "Error in configuration at
a.p.p._.s.e.r.v.i.c.e._.c.o.n.f.i.g._.f.i.l.e.s'" when the config
option has the wrong data type.

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/15614.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.82.0 where the error message displayed when validation of the `app_service_config_files` config option fails would be incorrectly formatted.

synapse/config/appservice.py

@@ -36,11 +36,10 @@ class AppServiceConfig(Config):
         if not isinstance(self.app_service_config_files, list) or not all(
             type(x) is str for x in self.app_service_config_files
         ):
-            # type-ignore: this function gets arbitrary json value; we do use this path.
             raise ConfigError(
                 "Expected '%s' to be a list of AS config files:"
                 % (self.app_service_config_files),
-                "app_service_config_files",
+                ("app_service_config_files",),
             )
 
         self.track_appservice_user_ips = config.get("track_appservice_user_ips", False)