[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/services/tree/replicator.go

@@ -59,7 +59,7 @@ func (s *Service) localReplicationWorker(ctx context.Context) {
 			err := s.forest.TreeApply(ctx, op.cid, op.treeID, &op.Move, false)
 			if err != nil {
 				s.log.Error(ctx, logs.TreeFailedToApplyReplicatedOperation,
-					zap.String("err", err.Error()))
+					zap.Error(err))
 			}
 			span.End()
 		}
@@ -155,7 +155,7 @@ func (s *Service) replicateLoop(ctx context.Context) {
 			err := s.replicate(op)
 			if err != nil {
 				s.log.Error(ctx, logs.TreeErrorDuringReplication,
-					zap.String("err", err.Error()),
+					zap.Error(err),
 					zap.Stringer("cid", op.cid),
 					zap.String("treeID", op.treeID))
 			}