Description of the issue

The error "ERROR: network network_main_net id ... has active endpoints" isn't particularly useful and blocks docker-compose down.

Details

I have multiple docker-compose.yml files that look like this:

version: '2.3'
services:
  someservice:
    networks:
    - networks_main_net
    ...

networks:
  network_main_net:
    name: network_main_net

So far, this is the only way to have a shared network over multiple docker-compose.yml groups which doesn't involve some hackery with a special network-owning folder that needs to be uped first.

However, for all of those containers, docker-compose down && docker-compose up -d is broken:

# docker-compose down && docker-compose up -d
Stopping mail ... done
Removing mail ... done
Removing network network_main_net
ERROR: network network_main_net id 6aaf7ec207e435452261aa3f408813c1197d5a5080288d70e4886373427c9fe7 has active endpoints

Since the error isn't actually useful to me and certainly not something I would want it to abort over (of course there are still active endpoints - that's the point of sharing the network!) I suggest that it is converted into a warning.

Context information (for bug reports)

# docker-compose version
docker-compose version 1.21.2, build a133471
docker-py version: 3.3.0
CPython version: 3.6.5
OpenSSL version: OpenSSL 1.0.1t  3 May 2016

# docker version
Client:
 Version:	17.12.1-ce
 API version:	1.35
 Go version:	go1.10.1
 Git commit:	7390fc6
 Built:	Wed Apr 18 01:23:11 2018
 OS/Arch:	linux/amd64

Server:
 Engine:
  Version:	17.12.1-ce
  API version:	1.35 (minimum version 1.12)
  Go version:	go1.10.1
  Git commit:	7390fc6
  Built:	Wed Feb 28 17:46:05 2018
  OS/Arch:	linux/amd64
  Experimental:	false

Steps to reproduce the issue

1. Use shared network alike to networks.some_name.name: some_name (used by a container as services.myservice.networks: ["some_name"] as shown above)
2. Set up 2+ docker-compose.yml groups that all share this same network name
3. Do docker-compose up -d for all the groups sharing this network
4. Do docker-compose down on one of them

Observed result

error message that isn't particularly surprising or relevant, and exit code non-zero

Expected result

warning (or no network-related output at all) and exit code 0

Stacktrace / full error message

Removing network network_main_net
ERROR: network network_main_net id 6aaf7ec207e435452261aa3f408813c1197d5a5080288d70e4886373427c9fe7 has active endpoints

Additional information

Ubuntu 18.04 LTS