Message style fixes

src/bin/psql/command.c

src/bin/psql/help.c

src/interfaces/ecpg/ecpglib/error.c

src/interfaces/ecpg/ecpglib/prepare.c

src/interfaces/ecpg/preproc/ecpg.trailer

src/interfaces/ecpg/test/expected/sql-declare.stderr

src/interfaces/libpq/fe-connect.c