livefs build doesn't show useful error if "project" metadata is missing

[Robin Winslow]

From cjwatson:

> - I can't get any builds to do anything other than fail:
> [5]https://launchpad.net/~imagebuild/+livefs/ubuntu/bionic/ubuntu-cpc/, (but I've only tried with Toto snap)

Well, it would certainly be nice if you got a better error message here
(please file a bug about that). But the problem here is that your
metadata dictionary doesn't include the "project" field, which in this
case should probably be set to "ubuntu-cpc" but in others should perhaps
be "ubuntu-core". You can either set this in "metadata" when creating
the livefs object (as
https://launchpad.net/~ubuntu-cdimage/+livefs/ubuntu/focal/cpc does) or
in "metadata_override" when creating the livefs_build object; the two
dictionaries are merged together.