Make errors in finding current version visible

Was silent in https://github.com/openfisca/country-template/actions/runs/8810580013, which led to failed merge in #139

.github/is-version-number-acceptable.sh

@@ -14,14 +14,20 @@ fi
 
 current_version=$(grep '^version =' pyproject.toml | cut -d '"' -f 2)  # parsing with tomllib is complicated, see https://github.com/python-poetry/poetry/issues/273
 
+if [[ ! $current_version ]]
+then
+    echo "Error getting current version"
+    exit 1
+fi
+
 if git rev-parse --verify --quiet $current_version
 then
     echo "Version $current_version already exists in commit:"
     git --no-pager log -1 $current_version
     echo
     echo "Update the version number in pyproject.toml before merging this branch into main."
     echo "Look at the CONTRIBUTING.md file to learn how the version number should be updated."
-    exit 1
+    exit 2
 fi
 
 if ! $(dirname "$BASH_SOURCE")/has-functional-changes.sh | grep --quiet CHANGELOG.md