Non existant dependency error message is super unhelpful

[city41]

If a dependency can't be met, the only output is "Non-existant dependency"

This is because the way dependency resolution is implemented, it's difficult to know what dependency is missing. Rewriting prioritize completely is probably the real answer, it's getting unwieldy anyway.

short term fix: output all of the remaining unmet dependencies in the error message.

[city41]

The "fix" for this is still not great, but better than nothing. I'll look into truly fixing this for real when I get some down time.