Re: Mention column name in error messages

Okay, so I have reworked the patch a bit and finished with the
attached, adapting the context message to give more information. I
have noticed as well a bug in the patch: the context callback was set
before checking if the column used for transformation is checked on
being a system column or not, the problem being that the callback
could be called without the fields set.

I have updated the regression tests that I found, the main portion of
the patch being dedicated to that and being sure that all the
alternate outputs are correctly refreshed. In this case int8, float4,
float8, xml and contrib/citext are the ones impacted by the change
with alternate outputs.

I am passing that down to a committer for review. The patch looks
large, but at 95% it involves diffs in the regression tests,
alternative outputs taking a large role in the bloat.