Avoid waiting on a container that died (#3631)

This is happening for me locally with mofuw.  Its CMD line fails:

  /bin/sh: 1: ./start-servers.sh: not found

and then the toolset waits around for a long time for it to be running.
With the change, it detects that it has failed quickly.

toolset/benchmark/benchmarker.py

@@ -158,6 +158,8 @@ class Benchmarker:
             max_sleep = 60
             accepting_requests = False
             while not accepting_requests and slept < max_sleep:
+                if not docker_helper.server_container_exists(self.config, container.id):
+                    break
                 accepting_requests = test.is_accepting_requests()
                 time.sleep(1)
                 slept += 1

toolset/utils/docker_helper.py

@@ -223,7 +223,11 @@ def stop(benchmarker_config=None,
                 container.stop()
     else:
         # Stop the running container
-        container.stop()
+        try:
+            container.stop()
+        except Exception:
+            # Suppress "No such container" errors
+            pass
 
     database_client = docker.DockerClient(
         base_url=benchmarker_config.database_docker_host)
@@ -235,7 +239,11 @@ def stop(benchmarker_config=None,
             ) > 0 and 'techempower' in container.image.tags[0] and 'tfb:latest' not in container.image.tags[0]:
                 container.stop()
     else:
-        database_container.stop()
+        try:
+            database_container.stop()
+        except Exception:
+            # Suppress "No such container" errors
+            pass
 
     client.containers.prune()
 
@@ -342,6 +350,19 @@ def test_client_connection(benchmarker_config, url):
     return True
 
 
+def server_container_exists(benchmarker_config, container_id_or_name):
+    '''
+    Returns True if the container still exists on the server.
+    '''
+    client = docker.DockerClient(
+        base_url=benchmarker_config.server_docker_host)
+    try:
+        client.containers.get(container_id_or_name)
+        return True
+    except:
+        return False
+
+
 def benchmark(benchmarker_config, script, variables, raw_file):
     '''
     Runs the given remote_script on the wrk container on the client machine.