Don't fail build if volume-clean fails.

This fix is admittedly simplistic.  It does not take into account any of the
other possible causes for an exception.  However, if you are using volume-clean,
chances are you're using volume-create, or many of the other goals provided by
DMP.  In that case, you will almost certainly receive errors pointing you at the
real problem.

src/main/java/io/fabric8/maven/docker/service/VolumeService.java

@@ -33,6 +33,14 @@ public String createVolume(VolumeConfiguration vc) throws DockerAccessException
    }
 
    public void removeVolume(String volumeName) throws DockerAccessException {
-      docker.removeVolume(volumeName);
+      try {
+	docker.removeVolume(volumeName);
+      } catch ( DockerAccessException dae ) {
+	// IGNORE
+	// If you're using volume-clean, the most likely cause for a failure
+	// is that the volume doesn't exist.  In that case, the build should
+	// not be failed.  For any other probable cause of failure, another
+	// goal will almost certainly point you at the real problem.
+      }
    }
 }