Once the run's context is cancelled, database/sql rolls an open
transaction back itself, so the rollback in WithTx returned
sql.ErrTxDone. WithTx treated any rollback error as fatal and called
os.Exit(1): an interrupted backup printed "FATAL: failed to rollback
transaction", exited 1 instead of 130 and skipped its deferred cleanup.
WithTx now ignores sql.ErrTxDone from the rollback and returns the
body's error alone. A rollback that really fails is returned together
with the body's error instead of ending the process; on a panic it is
dropped and the panic carries on. The cancel test checks that the
returned error carries no sql.ErrTxDone; the other tests make the
rollback really fail by running ROLLBACK inside the body.
Model: opus-5-5
sneak