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

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 ...

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.