Add status when pipeline cannot be paused

[emgrasmeder]

During provisioning / initialization of a pipeline(e.g. downloading an image), the pipeline cannot be paused. We should notify users that the pipeline cannot be paused, so they know to either stop the pipeline or retry pausing later.

A suggestion for the return status could be: unpausable/provisioning. The status code would be 422.

The actual internal state of bob should have the pipeline status listed as :initializing until the image pull is complete, after which it can be :running

[lispyclouds]

Implemented in 51152c8
cc @emgrasmeder