Unify error code in LibsqlError for local and remote

We currently have divergence in what error code means in local and remote
setups. Fix the consistency issue by making error code mean the basic one,
which is what the SQL over HTTP protocol returns.

However, add SQLite extended error codes to LibSqlError and populate that for
the local setup. We'll add that for remote too, but that requires a SQL over
HTTP protocol extension and server side change.