Please report errors via --progress as well

[Michael Vogt]

The new --progress=json support in s-i is great! However snappy also needs support for error reporting over this channel.

In the initial branch for this at https://code.launchpad.net/~mvo/ubuntu-system-image/json-output/+merge/251226
this was implemented in line 69 of the LP diff, i.e.:
"""
print(json.dumps({"type": "error", "msg": str(e)}))
"""