Validate DB schema in `check_database`
table_exists will only check if there's a table with that name, and returns true even if columns are wrong.
This is okay for now since I don't see how a table without the right columns could happen except if the user fucked with the DB manually, in which case he should know enough to fix it himself.
This will be a real problem if you ever change the schema in later versions. When that happens, use something like
peewee-validates to validate the schema.
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