Don't crash when a branch wasn't built correctly

managetests/instances.py

@@ -83,7 +83,13 @@ class InstancesManager:
             "run", "-g", "%s.py" % self.manager.gunicorn_config_file,
             "%s/%s" % (self.manager.caches_dir, branch.name),
         ]
-        process = subprocess.Popen(command)
+        try:
+            process = subprocess.Popen(command)
+        except FileNotFoundError:
+            print("managetests: error: can't start the %s branch: executable "
+                  "not found" % branch.name)
+            return
+
         self._processes[branch.name] = process
 
     def stop(self):