Ignore errors from lastInsertId()

lastInsertId() fails if the DB structure does not match Db module's assumption, for example, a table without tablename_id_seq sequence in PostgreSQL.

I agree with the opinion in #989, Db module is to be simple.
However, it is better to avoid causing error even if the database structure is unexpected.
So I suggest to ignore the exception for uncommon conditions.

This provides the same behavior as other RDBMS like MySQL for PostgreSQL.
I think this is good meeting ground between simplicity and stability.