Show better error message when pool doesn't exist for volume/image

[flaviostutz]

I run the following command "docker run -it --rm --volume-driver=rbd --name first --volume mypool/myimage:/mnt/foo ubuntu /bin/bash -c "echo -n 'Hello ' >> /mnt/foo/hello"

"mypool" didn't exist, so I had the following message: "docker: Error response from daemon: create mypool/myimage: VolumeDriver.Create: Unable to create Ceph RBD Image(myimage): exit status 2.
See 'docker run --help'."

Logs:

ceph-driver_1  | 2018/07/28 06:35:01 driver.go:687: INFO: Attempting to create new RBD Image: (mypool/myimage, %!s(int=20480), xfs)
ceph-driver_1  | 2018/07/28 06:35:01 utils.go:73: DEBUG: shWithTimeout: 2m0s, rbd, [--pool mypool --conf /etc/ceph/ceph.conf --id admin create --image-format 2 --size 20480 myimage]
ceph-driver_1  | 2018/07/28 06:35:01 utils.go:38: DEBUG: sh CMD: &{"/usr/bin/rbd" ["rbd" "--pool" "mypool" "--conf" "/etc/ceph/ceph.conf" "--id" "admin" "create" "--image-format" "2" "--size" "20480" "myimage"] [] "" <nil> <nil> <nil> [] %!q(*syscall.SysProcAttr=<nil>) %!q(*os.Process=<nil>) "<nil>" <nil> <nil> %!q(bool=false) [] [] [] [] %!q(chan error=<nil>) %!q(chan struct {}=<nil>)}
ceph-driver_1  | 2018/07/28 06:35:01 driver.go:203: ERROR: Unable to create Ceph RBD Image(myimage): exit status 2

I discovered the reason by running manually the command the driver was issuing:
"/usr/bin/rbd --pool mypool --conf /etc/ceph/ceph.conf --id admin create --image-format 2 --size 20480 myimage"
Now the log told me the pool didn't exist.