all: Make all error implementations pointer types (#6726)

This matches the convention of the stdlib and avoids ambiguity: when
customErr{} and &customErr{} both implement error, client code needs to
check for both.

Memory use should remain the same, since storing a non-pointer type in
an interface value still copies the value to the heap.

lib/db/backend/badger_backend.go

@@ -402,10 +402,10 @@ func wrapBadgerErr(err error) error {
 		return nil
 	}
 	if err == badger.ErrDiscardedTxn {
-		return errClosed{}
+		return &errClosed{}
 	}
 	if err == badger.ErrKeyNotFound {
-		return errNotFound{}
+		return &errNotFound{}
 	}
 	return err
 }