How to tell if there is an error message page in firefox

https://support.mozilla.org/z.

A competitor"s site is embedded in iframe. I hope to do something when the "your connection is not secure" page" appears.
how can I tell if there is an "error message page"?

Https html javascript firefox
Apr.11,2021

js should be able to listen for events corresponding to iframe, or will there be a status code?
anyway, the general idea is to listen for the event and then execute the processing.

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e9159a-47112.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e9159a-47112.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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