_refresh_store doesn't capture stderr in failure cases

[Adam Dyess]

The layer calls `snap refresh` but in the event of a failure doesn't log the reason for the failure.

```python
    hookenv.log("Refreshing {} from store".format(snapname))
    out = subprocess.check_output(cmd, stderr=subprocess.STDOUT)
    print(out)
```

Doesn't catch subprocess.CalledProcessError which contains valid stderr that isn't logged

https://pastebin.ubuntu.com/p/vWKbTd824p/

It is LIKELY that retrying the snap refresh would complete successfully. We could use tenacity similar to `snap._install_store` to retry 3 times with 10s apart. Lets log each retry

[Adam Dyess (addyess)]

Link to MR
https://code.launchpad.net/~addyess/layer-snap/+git/layer-snap/+merge/452021