Warn on failed validate of empty payload signature

If signature validation fails on an empty payload, append a note to the
end of the error message.

Updates #423

test/hooks.yaml.tmpl

@@ -150,3 +150,16 @@
 - id: warn-on-space
   execute-command: '{{ .Hookecho }} foo'
   include-command-output-in-response: true
+
+- id: empty-payload-signature
+  include-command-output-in-response: true
+  execute-command: '{{ .Hookecho }}'
+  command-working-directory: /
+  trigger-rule:
+    and:
+    - match:
+        parameter:
+          source: header
+          name: X-Hub-Signature
+        secret: mysecret
+        type: payload-hash-sha1