First bits of work on error message editing.

doc/src/sgml/sources.sgml

src/backend/catalog/pg_proc.c

src/backend/commands/dbcommands.c

src/backend/commands/functioncmds.c

src/backend/commands/opclasscmds.c

src/backend/commands/proclang.c

src/backend/commands/schemacmds.c

src/backend/commands/user.c

src/backend/parser/analyze.c

src/backend/parser/parse_coerce.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_func.c

src/backend/parser/parse_oper.c

src/backend/parser/scan.l

src/backend/utils/error/elog.c

src/backend/utils/fmgr/dfmgr.c

src/include/utils/elog.h

src/test/regress/expected/alter_table.out

src/test/regress/expected/create_type.out

src/test/regress/expected/privileges.out