fix: return 404 when joining non-existent room

Fixes #1443.

changelog.d/1579.bugfix

@@ -0,0 +1 @@
+Fixed error 500 when joining non-existent rooms. Contributed by @ezera.

src/api/client/membership/join.rs

@@ -785,6 +785,15 @@ async fn join_room_by_id_helper_local(
 	};
 
 	if servers.is_empty() || servers.len() == 1 && services.globals.server_is_ours(&servers[0]) {
+		if !services.rooms.metadata.exists(room_id).await {
+			return Err!(Request(
+				Unknown(
+					"Room was not found locally and no servers were found to help us discover it"
+				),
+				NOT_FOUND
+			));
+		}
+
 		return Err(error);
 	}
 

src/core/error/err.rs

@@ -77,6 +77,14 @@ macro_rules! err {
 		)
 	};
 
+	(Request($variant:ident($($args:tt)+), $status_code:ident)) => {
+		$crate::error::Error::Request(
+			$crate::ruma::api::client::error::ErrorKind::$variant,
+			$crate::format_maybe!($($args)+),
+			$crate::http::StatusCode::$status_code,
+		)
+	};
+
 	(Config($item:literal, $($args:tt)+)) => {{
 		let mut buf = String::new();
 		$crate::error::Error::Config($item, $crate::err_log!(buf, error, config = %$item, $($args)+))