[#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>

cmd/frostfs-node/tree.go

@@ -113,7 +113,7 @@ func initTreeService(c *cfg) {
 			// Ignore pilorama.ErrTreeNotFound but other errors, including shard.ErrReadOnly, should be logged.
 			c.log.Error(ctx, logs.FrostFSNodeContainerRemovalEventReceivedButTreesWerentRemoved,
 				zap.Stringer("cid", ev.ID),
-				zap.String("error", err.Error()))
+				zap.Error(err))
 		}
 	})