Improve "constraint must include all partitioning columns" message.

Improve "constraint must include all partitioning columns" message.

This formerly said "unique constraint must ...", which was accurate
enough when it only applied to UNIQUE and PRIMARY KEY constraints.
However, now we use it for exclusion constraints too, and in that
case it's a tad confusing.  Do what we already did in the errdetail
message: print the constraint_type, so that it looks like "UNIQUE
constraint ...", "EXCLUDE constraint ...", etc.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACJufxH6VhAf65Vghg4T2q315gY=Rt4BUfMyunkfRj0n2S9n-g@mail.gmail.com