VIP: enhanced errors

Hello all

I am working on new diagnostics fields in errors - CONSTRAINT_NAME,
SCHEMA_NAME, TABLE_NAME and COLUMN_NAME.

These fields is shown when verbosity mode is active. Actually this
works for table constraints, not null constraint and for RI
constraints.

postgres=# delete  from xxx;
ERROR:  23503: update or delete on table "xxx" violates foreign key
constraint "yyy_b_fkey" on table "yyy"
DETAIL:  Key (a)=(10) is still referenced from table "yyy".
LOCATION:  ri_ReportViolation, ri_triggers.c:3593
CONSTRAINT:  yyy_b_fkey
SCHEMA:  public
TABLE:  xxx
COLUMN:  a

These fields should be available from GET DIAGNOSTICS statement too -
It could be next step after support for stacked diagnostics.

I looked on column name identification for column constraints - and
there is not any possible identification - so I need a new column to
pg_constraint table - "attrid", that should to identify column for
column's constraint.

This patch is just concept. Final patch will be significantly longer -
we need to check any "ereport" call, as this most important
constraints are table and ri constraints.

Regards

Pavel Stehule