Match the lower case of the podman error message

[afbjorklund]

Turns out that Docker says "No such network",
but Podman says "no such network". Little nuances.

---

This will fix the "error 125" results when inspecting the network.

$ docker network inspect foo
[]
Error: No such network: foo
$ echo $?
1
$ sudo podman network inspect foo
[]
Error: error inspecting object: no such network foo
$ echo $?
125

Closes #12679 #12683

[k8s-ci-robot]

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