Make error more verbose

.github/workflows/publish_testpypi.yml

@@ -2,6 +2,7 @@
 ---
 name: test-n-build
 on:
+  workflow_dispatch:
   push:
     tags:
       - v*
@@ -77,7 +78,11 @@ jobs:
         working-directory: ./src/py/
     name: Upload release to TestPyPI
     needs: super-test
-    if: always() && !cancelled() && !failure()
+    if: |
+      always() &&
+      !cancelled() &&
+      !failure() &&
+      startsWith(github.ref, 'refs/tags/')
     runs-on: ubuntu-latest
     environment:
       name: testpypi

src/py/kaleido/_mocker.py

@@ -158,7 +158,7 @@ def _load_figures_from_paths(paths: list[Path]):
 args = parser.parse_args()
 
 if not Path(args.output).is_dir():
-    raise ValueError("Specified output must be existing directory.")
+    raise ValueError(f"Specified output must be existing directory. Is {args.output!s}")
 
 
 # Function to process the images