fix: raise errors when unable to locate the previous commit in local history (#1551)

Co-authored-by: GitHub Action <action@github.com>

src/commitSha.ts

@@ -426,8 +426,8 @@ export const getSHAForPullRequestEvent = async (
           github.context.payload.action &&
           github.context.payload.action === 'synchronize'
         ) {
-          core.warning(
-            'Unable to locate the remote branch head sha. Falling back to the previous commit in the local history.'
+          throw Error(
+            'Unable to locate the previous commit in the local history. Please ensure to checkout pull request HEAD commit instead of the merge commit. See: https://github.com/actions/checkout/blob/main/README.md#checkout-pull-request-head-commit-instead-of-merge-commit)'
           )
         } else {
           core.info(