chore(storage): Add trait object error variant to DatabaseError

[emhane]

This PR contains one of the bindings which is needed to use DatabaseError in a type safe way in l2 code, for example in the OP historical proofs workstream. This allows downcast to OP error enum as opposed to adding additional wrapper cursor traits to be able to propagate OP errors in a type safe way (matching on error strings out ruled).

• Adds variant wrapping trait object error to DatabaseError
• Removes redundant PartialEq impl for DatabaseError, this was almost exclusively used in tests
• Updates tests to match on errors or compare the error variant data instead or relying on PartialEq

Ref op-rs#388

[mattsse]

I think this would be quite useful, now that we rollout another db for index data

wdyt @Rjected