This will also report the error without having to download the `.crate`
first.

If installing multiple packages, this will also report it immediately,
rather than waiting for the other packages to be installed first.

This also offers us more flexibility in the error we report,
like suggesting more appropriate fixes.

The next step would be to also automatically install an MSRV compatible
version if compatible with the version req (rust-lang#10903).
This improved error message will still be useful if the MSRV compatible
version is outside of the version req.

I did this as the first step
- Helps people now, not needing to wait on `-Zmsrv-policy` to be stabilized
- Has fewer questions on how it should be done (or if it should be)