fix not proper log message

[ChanYiLin]

this fix the issue in #727
If the err is errNotExists, then we can simply print the information and return directly.

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This change is 

[ChanYiLin]

/assign @gaocegege
could you help me to check, thanks!

[coveralls]

Coverage increased (+12.9%) to 69.836% when pulling aa357c6 on ChanYiLin:master into 9e396a4 on kubeflow:master.

[gaocegege]

/lgtm
/ok-to-test
/approve
Thanks for your contribution!

[k8s-ci-robot]

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[lovejoy]

have you test this ？I tried to fix this like your code，but may cause some other problem.
大哥你试过吗，我本来想的也是这么改，但是貌似会有其他问题呢

[gaocegege]

@lovejoy Could you please explain what the problem is

[lovejoy]

All right ,I found the problem is i did a fix in function getTFJobFromKey look likes this code. But that have something wrong.
好吧，是我改的那个有问题。看起来像这个但不是。

[gaocegege]

@lovejoy Gotcha. Thanks for the issue 😄

[ChanYiLin]

I didn't change any code in the function getTFJobFromKey.
Actually getTFJobFromKey return errNotExists when the tfjob was deleted.
So I just add this condition into processNextWorkItem() function.