Another round of error message editing, covering backend/parser/.

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/parser/parse_agg.c

src/backend/parser/parse_clause.c

src/backend/parser/parse_coerce.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_func.c

src/backend/parser/parse_node.c

src/backend/parser/parse_relation.c

src/backend/parser/parse_target.c

src/backend/parser/parse_type.c

src/backend/utils/fmgr/dfmgr.c

src/include/utils/elog.h

src/test/regress/expected/aggregates.out

src/test/regress/expected/alter_table.out

src/test/regress/expected/arrays.out

src/test/regress/expected/cluster.out

src/test/regress/expected/copy2.out

src/test/regress/expected/domain.out

src/test/regress/expected/errors.out