Fail on yaml lint errors (#738)

* Fail on yaml lint errors

Fixes #737

* Fix yaml lint spec after enabling failure on YamlLint

* Fix syntax error in yaml_lint.rb

* Use fail_hook instead of fail for spec file

* Fix if/else syntax to use elsif

* Fix yaml_lint linter errors

Author: Gui13

lib/overcommit/hook/pre_commit/yaml_lint.rb

@@ -10,8 +10,10 @@ def run
 
       if result.success?
         :pass
+      elsif result.stdout.include?('error')
+        [:fail, result.stdout]
       else
-        return [:warn, result.stdout]
+        [:warn, result.stdout]
       end
     end
   end

spec/overcommit/hook/pre_commit/yaml_lint_spec.rb

@@ -34,9 +34,22 @@
       )
     end
 
-    it { should warn }
+    it { should fail_hook }
   end
 
+  context 'and has 1 error and 1 warning' do
+    let(:result) do
+      double(
+        success?: false,
+        stdout: <<-MSG
+file1.yaml:3:81: [error] line too long (253 > 80 characters) (line-length)
+file2.yml:41:81: [warning] missing document start "---" (document-start)
+      MSG
+      )
+    end
+
+    it { should fail_hook }
+  end
   context 'and has single suggestion for missing file header' do
     let(:result) do
       double(