MDEV-7775 Wrong error message (Unknown error) when idle sessions are killed after wait_timeout

restore the error message that was removed by mistake in ec38c1bb

mysql-test/r/wait_timeout_not_windows.result

+set global log_warnings=2;
+set @@wait_timeout=1;
+set global log_warnings=@@log_warnings;

mysql-test/t/wait_timeout_not_windows.test

+source include/not_windows.inc;
+
+#
+# MDEV-7775 Wrong error message (Unknown error) when idle sessions are killed after wait_timeout
+#
+set global log_warnings=2;
+connect (foo,localhost,root);
+set @@wait_timeout=1;
+sleep 2;
+connection default;
+let SEARCH_FILE=$MYSQLTEST_VARDIR/log/mysqld.1.err;
+let SEARCH_RANGE= -50;
+let SEARCH_PATTERN= Aborted.*Got timeout reading communication packets;
+source include/search_pattern_in_file.inc;
+set global log_warnings=@@log_warnings;

sql/sql_connect.cc

@@ -1092,7 +1092,8 @@ void end_connection(THD *thd)
   }
 
   if (!thd->killed && (net->error && net->vio != 0))
-    thd->print_aborted_warning(1, ER(ER_UNKNOWN_ERROR));
+    thd->print_aborted_warning(1,
+      thd->stmt_da->is_error() ? thd->stmt_da->message() : ER(ER_UNKNOWN_ERROR));
 }