Remove modified/base schema if initialization from gpkg fails

If user tries to init from gpkg, but for some reason the init fails (e.g. keeping behind empty "modified" schema), another try to do init will result in a confusing error (this was the case in #89):
```
The 'modified' schema exists but the base schema is missing
```

As a fix, we should remove partially created modified/base schemas so that we don't have half-initialized data in the database.