Rename `DatabaseError` to `OpenError`

[xpe]

DatabaseError is a confusing name. Its doc comment says "Errors related to opening a database", but the name suggests it covers all database operations.

I suggest renaming to OpenError since:

• The namespace already provides context (redb::OpenError)
• Its variants (DatabaseAlreadyOpen, RepairAborted, UpgradeRequired) are all open-time errors
• Distinguishes it clearly from Error (the general superset enum for all redb errors)

Currently, having both DatabaseError (specific to opening) and Error (general) is confusing. OpenError makes the distinction obvious and follows Rust conventions of keeping names concise when the namespace provides context.

This is a breaking change but would clarify the API.

[xpe]

If you like the idea but don't want an immediate breaking change, you could use a type alias with a deprecation warning; e.g.:

#[deprecated(since = "3.2", note = "use `OpenError` instead")]
pub type DatabaseError = OpenError;

pub enum OpenError {
    // ...
}