ci: Fail pipeline when coverage data cannot be uploaded.

If we simply emit a warning (the default), the pipeline will fail during the check step when trying to combine the coverage data. By changing this from warning to error, we fail the pipeline earlier and save time debugging issues.

Author: seifertm

.github/workflows/main.yml

@@ -89,6 +89,7 @@ jobs:
       with:
         name: coverage-python-${{ matrix.python-version }}
         path: .coverage.*
+        if-no-files-found: error
 
   check:
     name: Check