Nuitka should test for requirements (like dependency tree walker) before starting build

I am trying to get a new CI build process going and I forgot to include --assume-yes-for-downloads in my nuikta build step. Nuikta took many minutes (for compilation to complete) before failing the build (at the linking step) stating that the dependency walker was required and would need to be downloaded.

Additionally, I don't think the error message suggested adding the --assume-yes-for-downloads option. I only got an error at the step where it asks if it's OK to download.

So, this is two very related requests, please:

1. Check if downloadable requirements are satisfied (and do the prompting and exception raising) prior to starting the build, so that users know ASAP that their build will fail.

2. Add something to the error message about the --assume-yes-for-downloads when doing a CI/unattended build, since it took me a while to search for this option on the website docs.