I have had the same error for my own technical addon and by looking at the source code of the alerts script, error code 3 seems related to input parameters not being correctly parsed or value None (null), resulting in the alert script to quit.
If you are trying to add search results to the email message, use the $result.$ to add that specific field value. For example, if you want to add the value of the field user from your search results, this would be accomplished with $result.user$
Could you provide the exact Splunk version you are working with?
And is that the exact string returned?
I'd suggest perhaps narrowing down the issue by creating a new alert with a little selected as possible. Then add one thing at a time to it and see when it fails.