Are query activity errors accessible through API? These would be SQL errors that are only thrown at run time. Traditionally you need to reach out to Salesforce Support to get access to these errors and I'm hoping they're exposed through API.
SFMC Partners used to have access to a server error log which was amazingly helpful for a thousand reasons, but it was discontinued when old ET support infrastructure was assimilated into Salesforce.
I've heard that some time in the future, query run-time errors will be visible in the interface.
That'll be the day.
It's in everyone's best interest that these kinds of basic development and debugging tools be available to those attempting to build complex solutions in the SFMC. It'd trade all of the WYSIWYG, drag-and-drop, eye-candy features for access to the error log.
Edit 2016-10-04: Yes, some query runtime errors are showing up in Automation Studio. It's still not has helpful as the error log.