correct docker.Run error handling

[BenTheElder]

fixes #53
/kind bug

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[BenTheElder]

checking err should occur before checking the output! fixed here

[BenTheElder]

FYI @mitar sorry about that, I introduced this bug in #51 yesterday.

[BenTheElder]

Now if you kind create twice you will at least get:

$ kind create
INFO[1002-23:23:05] Image: kindest/node:v1.11.3 present locally
INFO[1002-23:23:05] Running: /usr/local/bin/docker [docker run -d --privileged --security-opt seccomp=unconfined --tmpfs /tmp --tmpfs /run --tmpfs /var/lib/docker:exec -v /lib/modules:/lib/modules:ro --hostname kind-1-control-plane --name kind-1-control-plane --label io.k8s.sigs.kind.cluster=1 --expose 6443 --publish-all --entrypoint=/usr/local/bin/entrypoint kindest/node:v1.11.3 /sbin/init]
ERRO[1002-23:23:05] docker: Error response from daemon: Conflict. The container name "/kind-1-control-plane" is already in use by container "26a2d8a696d57f919443ec6fe850627d1809185badb76fb74e1cbea3e11f8cf6". You have to remove (or rename) that container to be able to reuse that name.
ERRO[1002-23:23:05] See 'docker run --help'.
FATA[1002-23:23:05] Failed to create cluster: docker run error: exit status 125

Next step is #39 and #23 to give nice, quieter output about resources from a previous cluster existing for this instead.

[krzyzacy]

/lgtm