Enhancement: Make s3 backend readError logic more robust

[wei840222]

for some s3 compatible storage backend, it may has differnt error messsage when object not exist.
I think using minio.error code for judge the object is not exist is more robust.

[CLAassistant]

All committers have signed the CLA.

[mapno]

Hi! Thanks for the contribution. Is there any difference between the current check err.Error() == s3KeyDoesNotExist and checking the error code for the same 'no key' error? It's being checked in both modified methods.

[mapno]

Sorry, I re-read your description and saw the explanation for some s3 compatible storage backend, it may has different error message . Then, would it make sense to just check the code?

[wei840222]

I think it is make sense to just check the code.
If possible, I can submit another commit to make corrections.

[wei840222]

@mapno I've changed.

[mapno]

LGTM, nice!

[wei840222]

It seems need approved for run CI.

[joe-elliott]

Thanks for the contribution, wei. We will wait til we cut 1.1 to merge.

[wei840222]

Got it, thx