snap find returns "no snaps found" when there is no network

[Michael Vogt]

If there is no network `snap find` will not return a useful result:
```
ubuntu@localhost:/etc$ ifconfig
lo Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING MTU:65536 Metric:1
          RX packets:160 errors:0 dropped:0 overruns:0 frame:0
          TX packets:160 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1
          RX bytes:12800 (12.8 KB) TX bytes:12800 (12.8 KB)

ubuntu@localhost:/etc$ snap find
error: no snaps found
```

It should return that it can not talk to the store instead.