Improve actionability of "resolver = 2" error

[alice-i-cecile]

Bevy users have been getting an error that looks like:

error: DX12 API enabled on non-Windows OS. If your project is not using resolver="2" in Cargo.toml, it should.

This is not very actionable or accessible for new users. Ideally, this error could be made somewhat more verbose to explain how to do this (and mention that they could swap to Rust 2021 instead).

Motivated by bevyengine/bevy#3205

[yottapanda]

The docs that explain how to "fix" this: https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

[Imberflur]

In regards to Rust 2021, I would like to note that it is somewhat misleading with respect to workspaces since you will need to specify resolver="2" in the workspace Cargo.toml regardless of the Rust edition being used for the contained crates.

(edit: that is when using a "virtual" manifest)

[fintelia]

If you don't want to include a more complete description in the error message, including a link could still provide a hint of where to find more details. Either to the URL @thechubbypanda suggested, to this issue, or whatever

[alice-i-cecile]

Some users aren't seeing this error at all: I'm not entirely sure why, and can't reproduce locally on Windows 10 :(

[Imberflur]

Cargo added a warning when the resolver isn't explicitly specified in a workspace containing crates using newer editions: rust-lang/cargo#10910

Although, this probably doesn't completely address the issue since a user could still explicitly set resolver = 1. Also, if wgpu is already a dependency, I suspect the new warning would still get lost in the errors that are generated when the resolver is not 2.

[cwfitzgerald]

This isn't going to be a problem anymore as we shouldn't require it as of v0.16.0