Add custom panic hook

[XAMPPRocky]

This PR overrides the default panic hook, with our own that will catch when SPIR-V tools are not available and give you a specific error message about that, and in all other cases we use the normal ICE message with the URL redirected to this repo, and we print out rust-gpu's version.

fixes #51

SPIR-V Error Message

error: failed to run custom build command for `example-runner v0.1.0 (/Users/src/rust-gpu/examples/example-runner)`

Caused by:
  process didn't exit successfully: `/Users/src/rust-gpu/target/debug/build/example-runner-062276209715c6a1/build-script-build` (exit code: 1)
  --- stderr
     Compiling spirv-std v0.1.0 (/Users/src/rust-gpu/spirv-std)
     Compiling example-shader v0.1.0 (/Users/src/rust-gpu/examples/example-shader)

  error: Couldn't find `spriv-val` tool in PATH.

  error: Please ensure that you have `spirv-tools` installed on your system and available in your PATH.

  note: `rust-gpu` version 0.1.0

  error: could not compile `example-shader`

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

Normal ICE

error: failed to run custom build command for `example-runner v0.1.0 (/Users/src/rust-gpu/examples/example-runner)`

Caused by:
  process didn't exit successfully: `/Users/src/rust-gpu/target/debug/build/example-runner-062276209715c6a1/build-script-build` (exit code: 1)
  --- stderr
     Compiling spirv-std v0.1.0 (/Users/src/rust-gpu/spirv-std)
  thread 'rustc' panicked at 'explicit panic', rustc_codegen_spirv/src/lib.rs:563:5
  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/EmbarkStudios/rust-gpu/issues/new

  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 lib

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

  note: `rust-gpu` version 0.1.0
  error: could not compile `spirv-std`

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

[khyperia]

The custom handling with rustc_driver::report_ice is really good, though, thanks for finding that!

[XAMPPRocky]

@khyperia Okay, I've refactored the PR into a single function output_spirv_tool that handles outputting tool errors as rustc compile errors and reworked the two functions to use that.

Error Message

     Compiling spirv-std v0.1.0 (/Users/erin.power/src/rust-gpu/spirv-std)
     Compiling example-shader v0.1.0 (/Users/erin.power/src/rust-gpu/examples/example-shader)
  error: Couldn't find `spirv-val` SPRI-V tool in PATH.
    |
    = note: Please ensure that you have `spirv-tools` installed on your system and available in your PATH.

  error: aborting due to previous error

  error: could not compile `example-shader`

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

[khyperia]

Manually rebased and tested locally and tests pass - so merging this! 🎉