Improve error message when /api/bootstrap request fails

[SpaceK33z]

Right now you get a long stack trace if the request to backend fails:

Traceback (most recent call last):
   File "/usr/local/lib/python3.6/site-packages/requests/adapters.py", line 440, in send
     timeout=timeout
   File "/usr/local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 639, in urlopen
     _stacktrace=sys.exc_info()[2])
   File "/usr/local/lib/python3.6/site-packages/urllib3/util/retry.py", line 388, in increment
     raise MaxRetryError(_pool, url, error or ResponseError(cause))
 urllib3.exceptions.MaxRetryError: HTTPConnectionPool(host='backend', port=8000): Max retries exceeded with url: /api/bootstrap/ (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fc9fa8ebe48>: Failed to establish a new connection: [Errno 111] Connection refused',))

But a human error like "Request to http://localhost:8000/api/bootstrap failed" would suffice.