Modifies a column's values in existing rows of a table or view. Restriction: You cannot use UPDATE on a table accessed via an engine that does not support UPDATE processing. You can update one or more ...

The update statement will return how many rows where modified, if that number is 0, you know that some other node got there first.

I'd like to know that a NULL is going to be inserted into a non-nullable field on one row via RAISEERROR, skip that row's update, and then allow the SQL statement to continue.