Add an error message when Docker container is not ready to be started. Ref #1281.

gns3server/compute/docker/docker_vm.py

@@ -352,7 +352,11 @@ class DockerVM(BaseNode):
     def start(self):
         """Starts this Docker container."""
 
-        state = yield from self._get_container_state()
+        try:
+            state = yield from self._get_container_state()
+        except DockerHttp404Error:
+            raise DockerError("Docker container '{name}' with ID {cid} does not exist or is not ready yet. Please try again in a few seconds.".format(name=self.name,
+                                                                                                                                                      cid=self._cid))
         if state == "paused":
             yield from self.unpause()
         elif state == "running":