Change DB interface to return errors

[melekes]

Low-level libs (db) should not decide whenever to panic or not. It's better to let dbs return an error and handle it in the calling code (say retry N times and maybe panic afterwards).

Pros:

• standard interface
• error handling is the same for every database (memdb, go_level_db, ...) => ease of customization in the future

Cons:

• we'll need to modify calling code

[melekes]

This will be a breaking change.