When running for example `clan networks list` we now get a reasonable
error message, instead of:
```
Traceback (most recent call last):
File "/nix/store/8ygq8bfxqydk2917mmg32wy9wb0qzzzd-clan-cli/bin/.clan-wrapped", line 9, in <module>
sys.exit(main())
~~~~^^
File "/nix/store/8ygq8bfxqydk2917mmg32wy9wb0qzzzd-clan-cli/lib/python3.13/site-packages/clan_cli/cli.py", line 516, in main
args.func(args)
~~~~~~~~~^^^^^^
File "/nix/store/8ygq8bfxqydk2917mmg32wy9wb0qzzzd-clan-cli/lib/python3.13/site-packages/clan_cli/network/list.py", line 12, in list_command
networks = networks_from_flake(flake)
File "/nix/store/8ygq8bfxqydk2917mmg32wy9wb0qzzzd-clan-cli/lib/python3.13/site-packages/clan_lib/network/network.py", line 106, in networks_from_flake
networks_ = flake.select("clan.exports.instances.*.networking")
^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'select'
```
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