Register exitcode output of the action

action.yml

@@ -35,6 +35,10 @@ inputs:
       $GITHUB_WORKSPACE. By default, Darker is run in $GITHUB_WORKSPACE.
     required: false
     default: '.'
+outputs:
+  exitcode:
+    description: "Exit code of Darker"
+    value: steps.darker.outputs.exitcode
 branding:
   color: "black"
   icon: "check-circle"
@@ -49,6 +53,7 @@ runs:
         pip install pip-requirements-parser
       shell: bash
     - name: Run Darker
+      id: darker
       run: |
         # Exists since using github.action_path + path to main script doesn't
         # work because bash interprets the backslashes in github.action_path