Since it appears that v10 is going to move the goalposts by some amount
in terms of where you can and can't invoke set-returning functions,
arrange for the executor's "set-valued function called in context that
cannot accept a set" errors to include a syntax position if possible,
pointing to the specific SRF that can't be called where it's located.

The main bit of infrastructure needed for this is to make the query source
text accessible in the executor; but it turns out that commit 4c728f38
already did that.  We just need a new function executor_errposition()
modeled on parser_errposition(), and we're ready to rock.

While experimenting with this, I noted that the error position wasn't
properly reported if it occurred in a plpgsql FOR-over-query loop,
which turned out to be because SPI_cursor_open_internal wasn't providing
an error context callback during PortalStart.  Fix that.

There's a whole lot more that could be done with this infrastructure
now that it's there, but this is not the right time in the development
cycle for that sort of work.  Hence, resist the temptation to plaster
executor_errposition() calls everywhere ... for the moment.

Discussion: https://postgr.es/m/5263.1492471571@sss.pgh.pa.us