Emit a better error when using mio on wasm

[Darksonn]

This configuration currently triggers a really bad error. See #1850 and #1681 for more info.

[Thomasdezeeuw]

Thanks @Darksonn, I hope it will work because it's usually printed before hundred of lines of output for the actual errors. I really wish that a version of compile_error existed that meant stop compiling the rest of the file, but this is the best we can do for now.