[bootstrap] Give a better error when trying to run a path with no registered step

[jyn514]

Before:

thread 'main' panicked at 'error: no rules matched invalid', src/bootstrap/builder.rs:287:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After:

error: no `check` rules matched 'invalid'
help: run `x.py check --help --verbose` to show a list of available paths
note: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

I think probably we should say something like "in src/bootstrap/builder.rs" here, though I guess it's not that hard to find:

$ rg 'describe!'
src/bootstrap/builder.rs
494:            Kind::Build => describe!(
525:            Kind::Check => describe!(
536:            Kind::Test => describe!(
591:            Kind::Bench => describe!(test::Crate, test::CrateLibrustc),
592:            Kind::Doc => describe!(
612:            Kind::Dist => describe!(
640:            Kind::Install => describe!(
654:            Kind::Run => describe!(run::ExpandYamlAnchors, run::BuildManifest, run::BumpStage0),

So maybe alright for now, and better short for such a specific message anyway.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 3a800bf has been approved by Mark-Simulacrum