libsql: Fix bootstrap error handling (#2148)

This fixes the following issues:

- If we fail to boostrap at database open, return an error instead of
allowing local files to be in inconsistent state. Note that we ignored
boostrap errors to avoid probing the remote for generation when offline,
but that I fixed now in a different way.
- If we have a database file, but no metadata, return an error instead
of allowing inconsistent state.