Fix logic for adding "parallel worker" context line to worker errors.

The previous coding here was capable of adding a "parallel worker" context
line to errors that were not, in fact, returned from a parallel worker.
Instead of using an errcontext callback to add that annotation, just paste
it onto the message by hand; this looks uglier but is more reliable.

Discussion: <19757.1472151987@sss.pgh.pa.us>