Populate artifacts on gh actions

.github/workflows/build.yml

@@ -10,15 +10,19 @@ jobs:
       matrix:
         include:
           - os: ubuntu-latest
+            artifact: "linux-x64"
             ocaml-compiler: "ocaml-variants.5.2.0+options,ocaml-option-flambda"
             triplet: "x64-linux"
           - os: windows-latest
+            artifact: "windows-x64"
             ocaml-compiler: "ocaml-variants.5.2.0+options,ocaml-option-flambda"
             triplet: "x64-mingw-static"
           - os: macos-latest
+            artifact: "macos-arm64"
             ocaml-compiler: "ocaml-variants.5.2.0+options,ocaml-option-flambda"
             triplet: "arm64-osx"
           - os: macos-13
+            artifact: "macos-x64"
             ocaml-compiler: "ocaml-variants.5.2.0+options,ocaml-option-flambda"
             triplet: "x64-osx"
     defaults:
@@ -73,6 +77,13 @@ jobs:
 
       - run: opam exec -- opam install . --deps-only --with-test
       - run: opam exec -- dune build --verbose
+      - if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: odiff-${{ matrix.artifact }}.exe
+          path: _build/default/bin/ODiffBin.exe
+          retention-days: 14
+
       - run: opam exec -- dune exec ODiffBin -- --version 
       - run: opam exec -- dune runtest
 
@@ -94,4 +105,3 @@ jobs:
 
       - name: e2e test
         run: npm test
-

bin/Main.ml

@@ -7,6 +7,10 @@ let getIOModule filename =
   | ".jpg" | ".jpeg" -> (module ODiffIO.Jpg.IO : ImageIO)
   | ".bmp" -> (module ODiffIO.Bmp.IO : ImageIO)
   | ".tiff" -> (module ODiffIO.Tiff.IO : ImageIO)
+  | "" ->
+      failwith
+        ("Usage: " ^ Sys.argv.(0)
+       ^ " <base_image_path> <image_to_compare_path> <output_png_path>")
   | f -> failwith ("This format is not supported: " ^ f)
 
 type 'output diffResult = { exitCode : int; diff : 'output option }