add warning to vg augment -i for nameless paths

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• vg augment -i warns when it is told to embed a path without a name

Description

Resolves #500 (the -i option moved from vg mod to vg augment in the interim). I could easily be convinced that this should be an error instead.

[adamnovak]

I don't think there's anything about generic paths that says the empty string can't be a path name. But it is indeed kind of an Air Bud situation.

[jltsiren]

If we want to maintain compatibility with GFA, generic path names must be non-empty, as they have to match regex [!-)+-<>-~][!-~]*.

[adamnovak]

Hmm. Unless we want to fail at GFA export, maybe we should indeed make this warning an error.