Unify error code in LibsqlError for local and remote (#337)

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.