Improve PL/Tcl errorCode facility by providing decoded name for SQLSTATE.

We don't really want to encourage people to write numeric SQLSTATEs in
programs; that's unreadable and error-prone.  Copy plpgsql's infrastructure
for converting between SQLSTATEs and exception names shown in Appendix A,
and modify examples in tests and documentation to do it that way.

src/backend/utils/errcodes.txt

@@ -15,6 +15,9 @@
 #   src/pl/plpgsql/src/plerrcodes.h
 #      a list of PL/pgSQL condition names and their SQLSTATE codes
 #
+#   src/pl/tcl/pltclerrcodes.h
+#      the same, for PL/Tcl
+#
 #   doc/src/sgml/errcodes-list.sgml
 #      a SGML table of error codes for inclusion in the documentation
 #