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>
