[#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/object/delete/delete.go

@@ -36,7 +36,7 @@ func (exec *execCtx) execute(ctx context.Context) error {
 	exec.log.Debug(ctx, logs.ServingRequest)
 
 	if err := exec.executeLocal(ctx); err != nil {
-		exec.log.Debug(ctx, logs.OperationFinishedWithError, zap.String("error", err.Error()))
+		exec.log.Debug(ctx, logs.OperationFinishedWithError, zap.Error(err))
 		return err
 	}