abstract possible database errors

[Erik1000]

Since we can't expose sqlx' errors because we also have indexed db, we need to abstract that kind of errors.

Take a look at sqlx' errors enum and figure out which variants are worth to include. Also, maybe we should have a look at the IndexedDB API for errors that can occur there.

[Erik1000]

• I/O Error: if the sqlite database file is inaccessible, the sqlite connection string is malformed or if there's an error with indexeddb e.g. it's an unsupported browser
• DuplicateError: if an entry already exists
• MigrationError: if the database migration fails (both sqlite and indexeddb)

Also, we might add an special I/O Error OpenError for the cases, where the user needs to give allow the webapp to use indexedDB. This would also cover a permission error on the file system with sqlite