Prevent users from stopping paused instances

[johannesduesing]

Describe the problem
Currently the registry allows users to call /stop on an instance which is in state Paused. However, Docker does not actually support this operation on paused containers, which causes the docker call to fail.

Describe the solution you would like
The registry should not execute a stop operation when the instance is in state Paused. Instead, the HTTP response should state a 400 Bad Request.

[johannesduesing]

Closed with #89