Error not being handled, instead pops up error prompt

I am tying to raise an error under an error handler to allow for a DB transaction to be rolled back when an attempt to create duplicate model in the database happens. However, the error handler does not handle this error and instead shows me the error prompt. Please advise.

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Which line is generating the error?
I tend to only begin a transaction before any changes are attempted with an additional boolean flag. Then in the error handler you use the flag to determine whether anything needs rolling back.

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This is essentially right after I start the transaction.

So if you step through, does it jump to the error handler at that line?
You could try leaving out the source parameter e.g.

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