Improve wording of some pg_upgrade failure reports.

Improve wording of some pg_upgrade failure reports.

Don't advocate dropping a whole table when dropping a column would
serve.  While at it, try to make the layout of these messages a
bit cleaner and more consistent.

Per suggestion from Daniel Gustafsson.  No back-patch, as we generally
don't like to churn translatable messages in released branches.

Discussion: https://postgr.es/m/2798740.1619622555@sss.pgh.pa.us