Currently failed sql queries are silently rolled back and user only can find information about this in log file. This may lead to unnoticed problems with missing data, etc.
As discussed with Alexei, a new counter item can be created, that is incremented each time an sql query gets failed on server or proxy.
This item with an appropriate trigger should be added to Template App Zabbix Server.
And a second item could be created, that has the content of last failed sql query.