This commit improves the ecpg's error message that commit f576de1db1 updated,
so that it gets rid of trailing period and uppercases the command name
in the error message.

Back-patch to v14 where the error message exists.

Author: Kyotaro Horiguchi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/20210819.170315.1413060634876301811.horikyota.ntt@gmail.com