fix: Use 404 instead of 400 (and include sender)

changelog.d/1308.feature.md

@@ -0,0 +1,2 @@
+Added unstable support for [MSC4406: `M_SENDER_IGNORED`](https://github.com/matrix-org/matrix-spec-proposals/pull/4406).
+Contributed by @nex

src/core/error/response.rs

@@ -85,7 +85,8 @@ pub(super) fn bad_request_code(kind: &ErrorKind) -> StatusCode {
 		| Unrecognized => StatusCode::METHOD_NOT_ALLOWED,
 
 		// 404
-		| NotFound | NotImplemented | FeatureDisabled => StatusCode::NOT_FOUND,
+		| NotFound | NotImplemented | FeatureDisabled | SenderIgnored { .. } =>
+			StatusCode::NOT_FOUND,
 
 		// 403
 		| GuestAccessForbidden