problems with pub.db.execSQL populating $dbMessage

We have a Production issue, with pub.db.execSQL

When I execute pub.db.execSQL this output variable $dbMessage is populated sometimes. And seems to be pub.db.execSQL dose not generate every time exception when error occur.
Maybe we should check value of $dbMessage, but I don�t know what to expect from $dbMessage.

There is no usable documentation about this output parameter.

I don understand logic behind $dbMessage.

Do you someone knows which kind of values can hold $dbMessage variable and in which cases is populated?