handle_error_when_executing_diff

[zhuguihua]

Executing diff can return exit code != 0, we should handle this error.

[aledbf]

@zhuguihua not sure we should return an error because of the diff exit code. This is just informational, not something affects the ingress controller. Returning an error means we will retry the build of the model

[zhuguihua]

@aledbf Thanks for your review, I did agree with you. The codes in this block are just only to show the nginx configuration differences. So I think errors of TempFile and WriteFile could not also be returned. I updated this patch, just give warning about differ command.

[aledbf]

/lgtm
/approve

[aledbf]

@zhuguihua thanks!

[k8s-ci-robot]

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