poll job status while waiting for single-user server to be reachable

[lexming]

Fixes #253

Poll job status while waiting to get the port back from the single-server to catch any failure in the job before the server launches.

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[ryanlovett]

This worked for me when I intentionally added an exit 1 to our form's prologue field.

This was to simulate early failures that jupyterhub will otherwise miss. When it happens, the user cannot start the server because jupyterhub thinks it is already running. This might not be a problem for deployments with short timeouts -- the user can just wait. However we have long timeouts to match those for non-interactive activity. This will be very helpful for us.

[opoplawski]

This would be very helpful.

[consideRatio]

This PR looks relevant even today, so I rebased this PR to resolve merge conflicts and get it to run against current tests. They are failing currently though =/

[consideRatio]

LGTM!

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