Convert panics to compiler errors

[khyperia]

There's a whole heck of a lot of panics, unwraps, and what-have-yous in rustc_codegen_spirv right now. Nearly all of them should probably be compiler errors instead.

Note this may require some rustc PRs, since right now, backends don't have access to span information. @eddyb may have some ideas around how to get span information, e.g. a bx.set_span method.

[khyperia]

bx.set_span: rust-lang/rust#77614

edit: has been merged, should be available in the next nightly - so, should be able to start on the conversion work.