Error if only `copy` exists and no `targets`

[jayvdb]

When only copy exists, i.e. targets is not used, the result is:

Error: No targets in .moban.yaml

[chfw]

hasn't thought targets would become optional. will code it in.

[jayvdb]

I've already done it.

It was a bit of a special case of importing files into a moban 'commons' repo (https://gitlab.com/coala/mobans/merge_requests/8), and may not be needed permanently if I find I need to template something.