Print error when --all-features is used

[philipc]

Fixes #232

[matprec]

This error is swamped among 1428 other errors, as the rest of the code still is type checked. For this to be visible, all modules would need to be #[cfg(not(feature = cargo-all))] in some way

[matprec]

Maybe something like this to reduce noise:

pub use modules::*;
#[cfg(not(feature = cargo-all))]
mod modules {
     pub mod foo;
     mod bar;
}

[philipc]

It's the first error. I'm not keen on adding further workarounds for this. rust-analyzer is disabling that setting anyway.

[matprec]

Fair enogh :) Looks good to me