make it more obvious when the rustc-pull failed

src/tools/miri/miri

@@ -121,10 +121,10 @@ rustc-pull)
     # Update rust-version file. As a separate commit, since making it part of
     # the merge has confused the heck out of josh in the past.
     echo "$FETCH_COMMIT" > rust-version
-    git commit rust-version -m "Preparing for merge from rustc"
+    git commit rust-version -m "Preparing for merge from rustc" || (echo "FAILED to commit rust-version file, something went wrong"; exit 1)
     # Fetch given rustc commit and note down which one that was
-    git fetch http://localhost:8000/rust-lang/rust.git@$FETCH_COMMIT$JOSH_FILTER.git
-    git merge FETCH_HEAD --no-ff -m "Merge from rustc"
+    git fetch http://localhost:8000/rust-lang/rust.git@$FETCH_COMMIT$JOSH_FILTER.git || (echo "FAILED to fetch new commits, something went wrong"; exit 1)
+    git merge FETCH_HEAD --no-ff -m "Merge from rustc" || (echo "FAILED to merge new commits, something went wrong"; exit 1)
     exit 0
     ;;
 rustc-push)