Add support for SQLSTATE error codes

Add support for SQLSTATE error codes

Postgres can report detailed information about an error using error codes.
See http://www.postgresql.org/docs/8.1/static/errcodes-appendix.html .
The current driver doesn't report the error, nor is it supported by the
QSqlError object.
The patch appends the error to the error message, helping applications to:
- handle different errors in a specific way
- show correct, translated error messages, independently on the language of the postgres installation

Change-Id: Ica3530ac33d3aaa9985e06f6c1f302ece9891033
Reviewed-by: Mark Brand <mabrand@mabrand.nl>