Capture exception messages of raised NotFound and BadRequest

* Partially reverts b4fdb72 and un-reverts 8f8df53 (oy vey!)

lib/sinatra/base.rb

@@ -609,6 +609,11 @@ module Sinatra
       status == 404
     end
 
+    # whether or not the status is set to 400
+    def bad_request?
+      status == 400
+    end
+
     # Generates a Time object from the given value.
     # Used by #expires and #last_modified.
     def time_for(value)
@@ -1133,12 +1138,15 @@ module Sinatra
 
       status(500) unless status.between? 400, 599
 
+      boom_message = boom.message if boom.message && boom.message != boom.class.name
       if server_error?
         dump_errors! boom if settings.dump_errors?
         raise boom if settings.show_exceptions? and settings.show_exceptions != :after_handler
       elsif not_found?
         headers['X-Cascade'] = 'pass' if settings.x_cascade?
-        body '<h1>Not Found</h1>'
+        body boom_message || '<h1>Not Found</h1>'
+      elsif bad_request?
+        body boom_message || '<h1>Bad Request</h1>'
       end
 
       res = error_block!(boom.class, boom) || error_block!(status, boom)

test/helpers_test.rb

@@ -26,6 +26,23 @@ class HelpersTest < Minitest::Test
     end
   end
 
+  describe 'bad_request?' do
+    it 'is true for status == 400' do
+      status_app(400) { bad_request? }
+      assert_body 'true'
+    end
+
+    it 'is false for status gt 400' do
+      status_app(401) { bad_request? }
+      assert_body 'false'
+    end
+
+    it 'is false for status lt 400' do
+      status_app(399) { bad_request? }
+      assert_body 'false'
+    end
+  end
+
   describe 'not_found?' do
     it 'is true for status == 404' do
       status_app(404) { not_found? }

test/routing_test.rb

@@ -54,7 +54,7 @@ class RoutingTest < Minitest::Test
 
     request = Rack::MockRequest.new(@app)
     response = request.request('GET', '/foo?bar=&bar[]=', {})
-    assert_equal 400, response.status
+    assert response.bad_request?
   end
 
   it "404s when no route satisfies the request" do
@@ -175,6 +175,32 @@ class RoutingTest < Minitest::Test
     assert_equal 404, status
   end
 
+  it "captures the exception message of a raised NotFound" do
+    mock_app {
+      get '/' do
+        raise Sinatra::NotFound, "This is not a drill"
+      end
+    }
+
+    get "/"
+    assert_equal "19", response["Content-Length"]
+    assert_equal 404, status
+    assert_equal "This is not a drill", response.body
+  end
+
+  it "captures the exception message of a raised BadRequest" do
+    mock_app {
+      get '/' do
+        raise Sinatra::BadRequest, "This is not a drill either"
+      end
+    }
+
+    get "/"
+    assert_equal "26", response["Content-Length"]
+    assert_equal 400, status
+    assert_equal "This is not a drill either", response.body
+  end
+
   it "uses 404 error handler for not matching route" do
     mock_app {
       not_found do