[Fix] Fix learn error propagation

src/libstat/backends/redis_backend.cxx

 #include "libserver/mempool_vars_internal.h"
 #include "fmt/core.h"
 
+#include "libutil/cxx/error.hxx"
+
 #include <string>
 #include <cstdint>
 #include <vector>
 	int id;
 	std::vector<std::pair<int, T>> *results = nullptr;
 	bool need_redis_call = true;
+	std::optional<rspamd::util::error> err;
 
 	using result_type = std::vector<std::pair<int, T>>;
 
 	}
 	else {
 		/* Error message is on index 3 */
+		const auto *err_msg = lua_tostring(L, 3);
+		rt->err = rspamd::util::error(err_msg, 500);
 		msg_err_task("cannot classify task: %s",
-					 lua_tostring(L, 3));
+					 err_msg);
 	}
 
 	return 0;
 rspamd_redis_finalize_process(struct rspamd_task *task, gpointer runtime,
 							  gpointer ctx)
 {
-	return TRUE;
+	auto *rt = REDIS_RUNTIME(runtime);
+
+	return !rt->err.has_value();
 }
 
 
 	}
 	else {
 		/* Error message is on index 3 */
-		msg_err_task("cannot learn task: %s",
-					 lua_tostring(L, 3));
+		const auto *err_msg = lua_tostring(L, 3);
+		rt->err = rspamd::util::error(err_msg, 500);
+		msg_err_task("cannot learn task: %s", err_msg);
 	}
 
 	return 0;
 rspamd_redis_finalize_learn(struct rspamd_task *task, gpointer runtime,
 							gpointer ctx, GError **err)
 {
+	auto *rt = REDIS_RUNTIME(runtime);
+
+	if (rt->err.has_value()) {
+		rt->err->into_g_error_set(rspamd_redis_stat_quark(), err);
+
+		return FALSE;
+	}
+
 	return TRUE;
 }