SSIS PL/SQL task hangs with message "Multiple-step OLE DB operation generated errors."
The reason the task was hanging was that I had run a separate update statement against MY_TABLE in a SQL Developer client window, and had not completed the transaction. Because the transaction was still open, the Execute SQL Task could not complete. Committing the transaction in SQL Developer allowed the SSIS package to complete, however, it still generated the same Warning message.

It appears the Warning message is harmless, though I still don't know how to interpret it.

Attention Microsoft SQL Server DBAs! The SQL Developer Oracle query window DOES NOT work like SQL Server Management Studio! DML queries are not implicit transactions, so and INSERT, UPDATE, OR DELETE statement leaves an open transaction unless you click the COMMIT or ROLLBACK button on the toolbar. Learn from my mistake.