On exitcode != 0, make sure to log the Docker error log

[robert-bor]

When a finite Docker process (such as docker ps) does not run because of a lack of privileges, it will quit without printing the error log.

The error.log, however, contains crucial information for the error, eg:

jenkins@some-slave01:~/jenkins_slave/workspace/somecustomer/somecustomer-core/snapshotter$ cat docker-std-err.log 
Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Get http://%2Fvar%2Frun%2Fdocker.sock/v1.25/containers/json: dial unix /var/run/docker.sock: connect: permission denied

The code to adjust is in DockerFiniteProcessRunner.execute.