coreos-installer download UX improvement ideas

[jlebon]

1. It should mention what part of the (arch, platform, format) triplet is missing. E.g.:

$ coreos-installer download -s next -p iso
Error: couldn't find platform 'iso' in stream metadata

2. It should either mention list-stream or even better print out the values available for the key that didn't match. E.g.:

$ coreos-installer download -s next -p metal -f img
Error: couldn't find format 'img' (available: 4k.raw.xz iso pxe raw.xz)

3. The format should have platform-sensitive defaults. E.g., if I specify -p gcp, it should just default to tar.gz. For platforms that have multiple formats (e.g. metal), we can choose to not default to any if preferred (but make sure we error out with the available formats as above).

Thought of these while playing with it during the FCOS 33 Test Day.

[dustymabe]

I've often thought a TUI (or even just shell) mode where you get a small menu of options and use arrow keys to select what you want would be nice.

[bgilbert]

There's some discussion here about not assuming FCOS by default in the install command, which is relevant for the download command too.