Enable QSqlError to handle alphanumeric error codes.

Some database systems (like PostgreSQL) use alphanumeric error codes.
Introduce a new method nativeErrorCode() which replaces number().
If the error code cannot be converted to int, number() will return 0.

Task-number: QTBUG-142
Change-Id: Ic7fba841737674b75c0c01c2263f51d2041da497
Reviewed-by: Mark Brand <mabrand@mabrand.nl>