fix: add check to multi-return option (#49)

Author: ttytm

2023/08/jacksonmowry.v

@@ -14,7 +14,9 @@ fn main() {
 }
 
 fn solver(filename string, part_2 bool) !i64 {
-	directions, nodes := os.read_file(filename)!.split_once('\n\n')
+	directions, nodes := os.read_file(filename)!.split_once('\n\n') or {
+		return error('Failed to derive nodes from "${filename}"')
+	}
 	node_lines := nodes.split_into_lines()
 	mut node_map := map[string]Node{}
 	mut working_nodes := []string{}