Improve Error Message When Missing SPIR-V tools

[XAMPPRocky]

Currently if you're missing the SPIR-V tools it gives you a No such file error along with an ICE. Both of these error messages don't really tell you how to solve the issue, and the ICE is a bit misleading since it's not a bug in Rust. It would be nice if it could exit gracefully with a message asking to install SPIR-V tools.

  thread 'rustc' panicked at 'spirv-val failed to execute: Os { code: 2, kind: NotFound, message: "No such file or directory" }', rustc_codegen_spirv/src/link.rs:158:25
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

  error: internal compiler error: unexpected panic

  note: the compiler unexpectedly panicked. this is a bug.

  note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

  note: rustc 1.49.0-nightly (8dae8cdcc 2020-10-12) running on x86_64-apple-darwin

  note: compiler flags: -Z unstable-options -Z codegen-backend=librustc_codegen_spirv.dylib -C opt-level=3 -C embed-bitcode=no --crate-type dylib

  note: some of the compiler flags provided by cargo are hidden

  error: could not compile `example-shader`

  To learn more, run the command again with --verbose.
  Error: BuildFailed

[khyperia]

Speaking of, the compiler is very much a WIP, and there are going to be a lot of ICEs, especially before #43 is complete. Is there a way to disable the standard ICE message, so we don't spam the rust-lang folks with our garbage? (not a question for @XAMPPRocky, just noting in general)

[repi]

Ah you mean something like if we build a binary of this and distribute it, and we have caused the ICE, that should be reported to this repo and not to the Rust one.