ctlmgr: catch create_subprocess_exec errors and retry

artiq/frontend/artiq_ctlmgr.py

@@ -47,8 +47,12 @@ class Controller:
             while True:
                 logger.info("Starting controller %s with command: %s",
                             name, command)
+                try:
                     process = yield from asyncio.create_subprocess_exec(*command.split())
                     yield from asyncio.shield(process.wait())
+                except FileNotFoundError:
+                    logger.warning("Controller %s failed to start", name)
+                else:
                     logger.warning("Controller %s exited", name)
                 logger.warning("Restarting in %.1f seconds", retry)
                 yield from asyncio.sleep(retry)