Unclear "failed to verify download" message when `gnupg` not installed

[bengesoff]

I'm using jill in a Docker image, and I changed the base image to a "slim" variant of the Debian image. I then started getting the following error:

----- Download Julia -----
downloading Julia release for 1.9.3-linux-x86_64
downloading from https://julialang-s3.julialang.org/bin/linux/x64/1.9/julia-1.9.3-linux-x86_64.tar.gz

finished downloading julia-1.9.3-linux-x86_64.tar.gz
downloading from https://julialang-s3.julialang.org/bin/linux/x64/1.9/julia-1.9.3-linux-x86_64.tar.gz.asc

finished downloading julia-1.9.3-linux-x86_64.tar.gz.asc
failed to verify 1.9.3-linux-x86_64 downloads
remove untrusted/broken files
False

I initially thought there must be some problem with that version of Julia, or the checksums, if it failed to verify. However after some messing around, I realised that gnupg wasn't installed in the "slim" variant, and that was causing the failure. I ran an apt-get install gnupg before calling Jill and it fixed it.

It is well documented in the README that this is required, but it might also be nice to display this in an error message too. Maybe it's getting swallowed by the except FileNotFound? For example, if the error message was "failed to verify, cannot find gnupg, please make sure it is installed", then it would be clearer.

Side note, this may be down to the fire library, but even with this error I think the CLI may have been returning exit status 0 as my Docker build continued as if it was successful. It would be nice if there was a way to return another exit status if there's an error

All in all, I managed to get it working, so thank you for creating this tool!