[per-OS Packages] Improve error message if package is uninstallable

[savil]

Summary

Instead of gating the entire per-OS packages feature due to some packages
not being installable on the user's platform for that particular version, I propose
this PR.

Here, we modify the error message to be less prescriptive. We explain the two
scenarios and let the user pick.

This is obviously not the ideal UX. But a good enough measure until we can
provide firmer guidance on which version of the package may work for this
platform?

How was it tested?

in axiom on mac, did devbox update and saw:

Ensuring packages are installed.

Installing package: gotty@latest.


Error: package gotty@latest cannot be installed on your platform x86_64-darwin.
If you know this package is incompatible with x86_64-darwin, then you could run `devbox add gotty@latest --exclude-platform x86_64-darwin` and re-try.
If you think this package should be compatible with x86_64-darwin, then it's possible this particular version is not available yet from the nix registry. You could try `devbox add` with a different version for this package.

[savil]

Current dependencies on/for this PR:

• main
  • PR [per-OS Packages] Improve error message if package is uninstallable #1387 👈

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[gcurtis]

I think this is a good approach so the user knows what's going on.