Method should never return explicit error type (only `error` interface)

[liran-funaro]

Assigning explicit error type (e.g., *ierrors.NotLeaderError) to an interface variable (e.g. var err error) will result in nil check to return false even if we assign a nil to it.
See https://go.dev/doc/faq#nil_error and https://go.dev/play/p/8HUKlSDPvCT

The following needs to change in bcdb/db.go:

DB

type DB interface {
	IsLeader() *ierrors.NotLeaderError
}

should be

type DB interface {
	IsLeader() error
}

TxProcessor

type TxProcessor interface {
	IsLeader() *ierrors.NotLeaderError
}

should be

type TxProcessor interface {
	IsLeader() error
}