Auto merge of rust-lang#3329 - RossSmyth:miri-bat-error, r=RalfJung

Fix miri.bat not bailing early on error

There is a logical error in the batch script that I did not think about. Currently, if the build step for miri-script fails, it will continue on and attempt to execute the next line. This will either lead to a file not found error, or running an old miri-script.

With this, it will bail upon erroring in the build step.

Author: bors

src/tools/miri/miri.bat

@@ -2,7 +2,10 @@
 :: Windows will not execute the bash script, and select this.
 @echo off
 set MIRI_SCRIPT_TARGET_DIR=%0\..\miri-script\target
-cargo build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml
+
+:: 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 build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml || 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