Diagnostic messages seem likely to help users diagnose root
causes more easily, so let's report them as errdetail.

Author: Thomas Munro
Reviewed-By: Ashutosh Bapat, Christoph Berg, Alvaro Herrera, Peter Eisentraut
Discussion: https://postgr.es/m/CAEepm=2_dA-SYpFdmNVwvKsEBXOUj=K4ooKovHmvj6jnMdt8dw@mail.gmail.com