Unhelpful error messages when submodules missing

[owensd]

I was pulling down some atpkg updates on my laptop, which had not been updated in a while so it didn't have the submodules yet.

When performing the build from the command line (atbuild v1.0), all I saw was this:

9:48:41 › atbuild -f build.atpkg check
enabling overlays ["atbuild.platform.osx", "atbuild.platform.mac"]
Could not load package file: NoSuchEntity
atbuild - Anarchy Tools Build Tool 1.0
https://github.com/AnarchyTools
© 2016 Anarchy Tools Contributors.

Usage:
atbuild [--toolchain (/toolchain/path | xcode)] [-f packagefile] [task] [--clean]
No tasks are available; run --help in a directory with a build.atpkg for project-specific help

I was really confused.

Turned out the issue was that I was missing the atfoundation submodule. After properly getting that, the build was working again.

We should provide some better error message here when the import-packages files cannot be found.

[owensd]

I see that there is this: Could not load package file: NoSuchEntity. However, that message is lost in the mini-wall-of-text, and also doesn't point to the the problem: atfoundation/build.atpkg was missing.

[dunkelstern]

It would be much clearer if we used the package manager for dependency resolution. See proposal here: #91

[drewcrawford]

See also AnarchyTools/atfoundation#9, which I believe would significantly improve this error

[dunkelstern]

AnarchyTools/atfoundation#9 has been merged, is there something that we can do here in addition to that?