helpers: handle flash[:error]

Errors generated by the `invisible-catcha` gem are reported as
`flash[:error]` (which differs from Rail's usual `flash[:alert]`).

We probably shouldn't use flash[:error] in our own codebase; but we can
handle its use in third-party libraries.

app/helpers/application_helper.rb

@@ -13,7 +13,7 @@ module ApplicationHelper
     case level
     when 'notice'
       class_names << 'alert-success'
-    when 'alert'
+    when 'alert', 'error'
       class_names << 'alert-danger'
     end
 

spec/helpers/application_helper_spec.rb

@@ -21,6 +21,7 @@ describe ApplicationHelper do
   describe "#flash_class" do
     it { expect(flash_class('notice')).to eq 'alert-success' }
     it { expect(flash_class('alert', sticky: true, fixed: true)).to eq 'alert-danger sticky alert-fixed' }
+    it { expect(flash_class('error')).to eq 'alert-danger' }
     it { expect(flash_class('unknown-level')).to eq '' }
   end