[#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/local_object_storage/engine/control.go

@@ -167,7 +167,7 @@ func (e *StorageEngine) close(ctx context.Context, releasePools bool) error {
 		if err := sh.Close(ctx); err != nil {
 			e.log.Debug(ctx, logs.EngineCouldNotCloseShard,
 				zap.String("id", id),
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 	}