SPI_execute error handling

The docs for SPI_execute at http://www.postgresql.org/docs/9.4/static/spi.html say:

"
Note that if a command invoked via SPI fails, then control will not be returned to your procedure. Rather, the transaction or subtransaction in which your procedure executes will be rolled back. (This might seem surprising given that the SPI functions mostly have documented error-return conventions. Those conventions only apply for errors detected within the SPI functions themselves, however.) It is possible to recover control after an error by establishing your own subtransaction surrounding SPI calls that might fail. This is not currently documented because the mechanisms required are still in flux.
"

so when my background worker executes "SELECT * FROM blah" and "blah" doesn't exist, i get:

2015-09-07 18:14:41 AEST [1958-44] ERROR: relation "blah" does not exist at character 15
2015-09-07 18:14:41 AEST [1958-45] QUERY: SELECT * FROM "blah"
2015-09-07 18:14:41 AEST [1889-18] LOG: worker process: tds handler (PID 1958) exited with exit code 1
2015-09-07 18:14:41 AEST [1889-19] LOG: unregistering background worker "tds handler"

And control is never returned to my worker, as expected.

How can I get control returned back to my worker so that I can give the client program a sensible error? I already create a transaction like StartTransactionCommand(), but then maybe that isn't the subtransation that the cryptic documentation above refers to??

thanks

James