persistent failure if initial manifest fetch fails

[bcressey]

On a new build host, I ran twoliter fetch without first configuring Docker to use the containerd snapshotter, or installing crane.

This failed, and the following directory was created under build:

build/external-kits/cache/sha256-b075d3764a0efc5f17bc4b27b040efa438310db30e9145588c32393ceeead9fa

Even after installing crane, I continued to see this error:

$ make
Found Twoliter v0.4.4 installed.
Skipping installation.
[2024-08-22T17:01:54Z TRACE twoliter::project] Looking for Twoliter.toml in '.'
[2024-08-22T17:01:54Z TRACE twoliter::project] This project does not have a Release.toml file (this is not a problem)
[2024-08-22T17:01:54Z DEBUG twoliter::project] Project file loaded from '/home/ec2-user/bottlerocket-playground/Twoliter.toml'
Error: Unable to read from '/home/ec2-user/bottlerocket-playground/build/external-kits/cache/sha256-b075d3764a0efc5f17bc4b27b040efa438310db30e9145588c32393ceeead9fa/index.json'

Caused by:
    No such file or directory (os error 2)
make: *** [Makefile:28: fetch] Error 1

If the directory exists, Twoliter assumes that index.json will be present, but it might not be. I'd expect it to recover from this error gracefully.