Fail a hook if it makes modifications. Resolves #285

testing/resources/modified_file_returns_zero_repo/bin/hook.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+for f in $@; do
+    echo modified > "$f"
+    echo "Modified: $f!"
+done

testing/resources/modified_file_returns_zero_repo/bin/hook2.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+echo $@

testing/resources/modified_file_returns_zero_repo/hooks.yaml

@@ -0,0 +1,10 @@
+-   id: bash_hook
+    name: Bash hook
+    entry: bin/hook.sh
+    language: script
+    files: ''
+-   id: bash_hook2
+    name: Bash hook
+    entry: bin/hook2.sh
+    language: script
+    files: ''