update postgresql error message parsing on foreign key

Error code: 23503

On "recent" postgresql (> 8?), error message is:
insert or update on table "$table" violates foreign key constraint
"$index"

I updated the error match to get the right table and index in both
cases.

Before this patch, sequelize failed badly with: `TypeError: Cannot read
property '1' of null`
Merge pull request #2393 from vvo/fix-pgsql-constraint-message

update postgresql error message parsing on foreign key