always render error templates as html

fixes CNVS-28641

test plan:
- access an invalid url with a non-html format, like:
  "/courses/X.png"
- it should only generate a "not found" error report, not a second error
  report about missing templates

Change-Id: I6aaf4ec6363bf229175d1cfd626a0c565d729531
Reviewed-on: https://gerrit.instructure.com/76825
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins
Product-Review: Simon Williams <simon@instructure.com>

Author: simonista

app/controllers/application_controller.rb

@@ -1187,6 +1187,7 @@ def render_rescue_action(exception, error, status, status_code)
       render template: template,
         layout: 'application',
         status: status_code,
+        formats: [:html],
         locals: {
           error: error,
           exception: exception,

spec/integration/application_spec.rb

@@ -195,4 +195,11 @@
       end
     end
   end
+
+  context "error templates" do
+    it "returns an html error page even for non-html requests" do
+      Canvas::Errors.expects(:capture).once.returns({})
+      get "/courses/blah.png"
+    end
+  end
 end