There is at least one more error message file, the one in the connector: libmariadb/libmariadb/ma_errmsg.c

It accounts for many remaining MySQL references:

mysql-test/main/crash_commit_before.result:ERROR HY000: Lost connection to MySQL server during query
mysql-test/main/delayed.result:ERROR HY000: Lost connection to MySQL server during query
mysql-test/main/myisam_crash_before_flush_keys.result:ERROR HY000: Lost connection to MySQL server during query
mysql-test/main/ssl_timeout.result:ERROR HY000: Lost connection to MySQL server during query
...

Please change error messages in ma_errmsg.c to say MariaDB instead of MySQL and wherever possible just "server". For example "Lost connection to MySQL server during query" would become "Lost connection to server during query" and "Unknown MySQL error" would become "Unknown MariaDB error".