assert_response: unwrap exceptions captured by ShowExceptions

If we get back a different response status from the one we were
expecting, and an exception was rescued, it's likely to be relevant.

Before:

Expected response to be a <200> but was a <500: Internal Server Error>.

After:

Expected response to be a <200> but was a <500: Internal Server Error>.

Exception while processing request: RuntimeError: foo
    <backtrace>

Author: matthewd

actionpack/lib/action_dispatch/testing/assertions/response.rb

@@ -87,15 +87,25 @@ def normalize_argument_to_redirection(fragment)
         end
 
         def generate_response_message(expected, actual = @response.response_code)
-          (+"Expected response to be a <#{code_with_name(expected)}>,"\
-          " but was a <#{code_with_name(actual)}>").concat(location_if_redirected).concat(response_body_if_short)
+          lambda do
+            (+"Expected response to be a <#{code_with_name(expected)}>,"\
+             " but was a <#{code_with_name(actual)}>").
+              concat(location_if_redirected).
+              concat(exception_if_present).
+              concat(response_body_if_short)
+          end
         end
 
         def response_body_if_short
           return "" if @response.body.size > 500
           "\nResponse body: #{@response.body}"
         end
 
+        def exception_if_present
+          return "" unless ex = @request&.env&.[]("action_dispatch.exception")
+          "\n\nException while processing request: #{Minitest::UnexpectedError.new(ex).message}\n"
+        end
+
         def location_if_redirected
           return "" unless @response.redirection? && @response.location.present?
           location = normalize_argument_to_redirection(@response.location)

actionpack/test/assertions/response_assertions_test.rb

@@ -136,6 +136,19 @@ def test_error_message_does_not_show_long_response_body
                    " but was a <400: Bad Request>"
         assert_match expected, error.message
       end
+
+      def test_error_message_shows_rescued_exception
+        @response = ActionDispatch::Response.new
+        @response.status = 500
+
+        @request = ActionDispatch::Request.new("action_dispatch.exception" => RuntimeError.new("example error"))
+
+        error = assert_raises(Minitest::Assertion) { assert_response 200 }
+        expected = "Expected response to be a <200: OK>,"\
+                   " but was a <500: Internal Server Error>\n\n"\
+                   "Exception while processing request: RuntimeError: example error\n"
+        assert_match expected, error.message
+      end
     end
   end
 end