Display exception messages using simple_format for a better and clearer

exception message.

actionpack/lib/action_dispatch/middleware/templates/rescues/_message_and_suggestions.html.erb

@@ -1,5 +1,7 @@
 <% if exception.respond_to?(:original_message) && exception.respond_to?(:corrections) %>
-  <h2><%= h exception.original_message %></h2>
+  <div class="exception-message">
+    <%= simple_format h(exception.original_message), { class: "message" }, wrapper_tag: "div" %>
+  </div>
   <%
     # The 'did_you_mean' gem can raise exceptions when calling #corrections on
     # the exception. If it does there are no corrections to show.
@@ -14,5 +16,7 @@
     </ul>
   <% end %>
 <% else %>
-  <h2><%= h exception.message %></h2>
+  <div class="exception-message">
+    <%= simple_format h(exception.message), { class: "message" }, wrapper_tag: "div" %>
+  </div>
 <% end %>

actionpack/lib/action_dispatch/middleware/templates/rescues/layout.erb

@@ -49,6 +49,18 @@
       line-height: 25px;
     }
 
+    .exception-message {
+      padding: 8px 0;
+    }
+
+    .exception-message .message{
+      margin-bottom: 8px;
+      line-height: 25px;
+      font-size: 1.5em;
+      font-weight: bold;
+      color: #C00;
+    }
+
     .details {
       border: 1px solid #D0D0D0;
       border-radius: 4px;

actionpack/test/dispatch/debug_exceptions_test.rb

@@ -629,7 +629,9 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
 
       # Assert correct error
       assert_response 500
-      assert_select "h2", /error in framework/
+      assert_select "div.exception-message" do
+        assert_select "div", /error in framework/
+      end
 
       # assert source view line is the call to method_that_raises
       assert_select "div.source:not(.hidden)" do
@@ -637,7 +639,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
       end
 
       # assert first source view (hidden) that throws the error
-      assert_select "div.source:first" do
+      assert_select "div.source" do
         assert_select "pre .line.active", /raise StandardError\.new/
       end
 
@@ -680,7 +682,9 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
 
       # Assert correct error
       assert_response 500
-      assert_select "h2", /Third error/
+      assert_select "div.exception-message" do
+        assert_select "div", /Third error/
+      end
 
       # assert source view line shows the last error
       assert_select "div.source:not(.hidden)" do
@@ -749,7 +753,9 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     get "/nil_annoted_source_code_error", headers: { "action_dispatch.show_exceptions" => true, "action_dispatch.logger" => logger }
 
     assert_select "header h1", /DebugExceptionsTest::Boomer::NilAnnotedSourceCodeError/
-    assert_select "#container h2", /nil annoted_source_code/
+    assert_select "#container div.exception-message" do
+      assert_select "div", /nil annoted_source_code/
+    end
   end
 
   test "debug exceptions app shows diagnostics for template errors that contain UTF-8 characters" do

railties/test/application/mailer_previews_test.rb

@@ -8,6 +8,7 @@ module ApplicationTests
   class MailerPreviewsTest < ActiveSupport::TestCase
     include ActiveSupport::Testing::Isolation
     include Rack::Test::Methods
+    include ERB::Util
 
     def setup
       build_app
@@ -299,7 +300,7 @@ module ApplicationTests
       app("development")
       get "/rails/mailers/notifier"
       assert_predicate last_response, :not_found?
-      assert_match "Mailer preview &#39;notifier&#39; not found", last_response.body
+      assert_match "Mailer preview &#39;notifier&#39; not found", h(last_response.body)
     end
 
     test "mailer preview email not found" do
@@ -329,7 +330,7 @@ module ApplicationTests
 
       get "/rails/mailers/notifier/bar"
       assert_predicate last_response, :not_found?
-      assert_match "Email &#39;bar&#39; not found in NotifierPreview", last_response.body
+      assert_match "Email &#39;bar&#39; not found in NotifierPreview", h(last_response.body)
     end
 
     test "mailer preview NullMail" do
@@ -385,7 +386,7 @@ module ApplicationTests
 
       get "/rails/mailers/notifier/foo?part=text%2Fhtml"
       assert_predicate last_response, :not_found?
-      assert_match "Email part &#39;text/html&#39; not found in NotifierPreview#foo", last_response.body
+      assert_match "Email part &#39;text/html&#39; not found in NotifierPreview#foo", h(last_response.body)
     end
 
     test "message header uses full display names" do