./miri: nicer error when building miri-script fails

Author: RalfJung

src/tools/miri/miri

@@ -3,5 +3,6 @@ set -e
 # Instead of doing just `cargo run --manifest-path .. $@`, we invoke miri-script binary directly. Invoking `cargo run` goes through
 # rustup (that sets it's own environmental variables), which is undesirable.
 MIRI_SCRIPT_TARGET_DIR="$(dirname "$0")"/miri-script/target
-cargo +stable build $CARGO_EXTRA_FLAGS -q --target-dir "$MIRI_SCRIPT_TARGET_DIR" --manifest-path "$(dirname "$0")"/miri-script/Cargo.toml
+cargo +stable build $CARGO_EXTRA_FLAGS -q --target-dir "$MIRI_SCRIPT_TARGET_DIR" --manifest-path "$(dirname "$0")"/miri-script/Cargo.toml || \
+  ( echo "Failed to build miri-script. Is the 'stable' toolchain installed?"; exit 1 )
 "$MIRI_SCRIPT_TARGET_DIR"/debug/miri-script "$@"