Following up on WebAssembly/wasi-sdk#210, this makes the trap message for `unreachable` traps more descriptive of what actually caused the trap, so that it doesn't sound like maybe Wasmtime itself executed a `unreachable!()` macro in Rust. Before: ``` wasm trap: unreachable wasm backtrace: [...] ``` After: ``` wasm trap: wasm `unreachable` instruction executed wasm backtrace: [...] ```