refactor: return more errors, add more tests (#679)

adding more tests
returns same message for short SHA format like GitHub Actions
refactor error checking
add more errors :)

pkg/runner/testdata/uses-github-full-sha/main.yml

@@ -0,0 +1,7 @@
+name: uses-github-root
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/hello-world-docker-action@b136eb8894c5cb1dd5807da824be97ccdf9b5423

pkg/runner/testdata/uses-github-short-sha/main.yml

@@ -0,0 +1,7 @@
+name: uses-github-root
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/hello-world-docker-action@b136eb8