Expose object name error fields in PL/pgSQL.

Specifically, permit attaching them to the error in RAISE and retrieving
them from a caught error in GET STACKED DIAGNOSTICS.  RAISE enforces
nothing about the content of the fields; for its purposes, they are just
additional string fields.  Consequently, clarify in the protocol and
libpq documentation that the usual relationships between error fields,
like a schema name appearing wherever a table name appears, are not
universal.  This freedom has other applications; consider a FDW
propagating an error from an RDBMS having no schema support.

Back-patch to 9.3, where core support for the error fields was
introduced.  This prevents the confusion of having a release where libpq
exposes the fields and PL/pgSQL does not.

Pavel Stehule, lexical revisions by Noah Misch.

src/test/regress/sql/plpgsql.sql

@@ -3262,6 +3262,38 @@ select raise_test();
 
 drop function raise_test();
 
+-- test passing column_name, constraint_name, datatype_name, table_name
+-- and schema_name error fields
+
+create or replace function stacked_diagnostics_test() returns void as $$
+declare _column_name text;
+        _constraint_name text;
+        _datatype_name text;
+        _table_name text;
+        _schema_name text;
+begin
+  raise exception using
+    column = '>>some column name<<',
+    constraint = '>>some constraint name<<',
+    datatype = '>>some datatype name<<',
+    table = '>>some table name<<',
+    schema = '>>some schema name<<';
+exception when others then
+  get stacked diagnostics
+        _column_name = column_name,
+        _constraint_name = constraint_name,
+        _datatype_name = pg_datatype_name,
+        _table_name = table_name,
+        _schema_name = schema_name;
+  raise notice 'column %, constraint %, type %, table %, schema %',
+    _column_name, _constraint_name, _datatype_name, _table_name, _schema_name;
+end;
+$$ language plpgsql;
+
+select stacked_diagnostics_test();
+
+drop function stacked_diagnostics_test();
+
 -- test CASE statement
 
 create or replace function case_test(bigint) returns text as $$