fixed linux build for relation completer

include/matador/sql/connection_pool.hpp

@@ -82,7 +82,10 @@ public:
       auto &conn = connection_repo_.back();
       idle_connections_.emplace(conn.first, &conn);
       // Todo: handle result
-      std::ignore = conn.second.open();
+      const auto result = conn.second.open();
+      if (!result) {
+        throw std::runtime_error("Failed to open connection");
+      }
       --count;
     }
   }