Auto merge of rust-lang#3703 - RossSmyth:ms-err, r=RalfJung

nicer batch file error when building miri-script fails

rust-lang/miri#3700 (comment)

src/tools/miri/miri.bat

@@ -5,7 +5,8 @@ set MIRI_SCRIPT_TARGET_DIR=%0\..\miri-script\target
 
 :: If any other steps are added, the "|| exit /b" must be appended to early
 :: return from the script. If not, it will continue execution.
-cargo +stable build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml || exit /b
+cargo +stable build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml ^
+  || echo Failed to build miri-script. Is the 'stable' toolchain installed? & exit /b
 
 :: Forwards all arguments to this file to the executable.
 :: We invoke the binary directly to avoid going through rustup, which would set some extra