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