Make non-zero ssh error codes less dramatic

[tstromberg]

Old output:

ls: cannot access '/xxx': No such file or directory
💣  Error creating SSH shell: Process exited with status 2

😿  Sorry that minikube crashed. If this was unexpected, we would love to hear from you:
👉  https://github.com/kubernetes/minikube/issues/new

New output:

ls: cannot access '/xxx': No such file or directory
ssh: Process exited with status 2

Closes #3700

[k8s-ci-robot]

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