Gracefully handle syntax (and other) errors when reloading config.rb.

With this change, config.rb modifications that result in an error will print the error to the logs but will not kill the server - the previous version of the application will still be running just fine, so that you can try and fix your config.rb without having to restart. The server no longer simply hangs with no message, which was the previous behavior. This fixes #702.

middleman-core/fixtures/padrino-helpers-app/config.rb

@@ -0,0 +1,7 @@
+helpers do
+  def title
+    content_tag :h1 do
+      "Title"
+    end
+  end
+end

middleman-core/lib/middleman-core/preview_server.rb

@@ -17,7 +17,7 @@ module Middleman
         @host = @options[:host] || Socket.gethostname
         @port = @options[:port] || DEFAULT_PORT
 
-        mount_instance
+        mount_instance(new_app)
         logger.info "== The Middleman is standing watch at http://#{host}:#{port}"
         logger.info "== Inspect your site configuration at http://#{host}:#{port}/__middleman/"
 
@@ -31,12 +31,17 @@ module Middleman
           # reloading later on.
           ::Middleman::Profiling.report("server_start")
 
+          loop do
             @webrick.start
 
             # $mm_shutdown is set by the signal handler
             if $mm_shutdown
               shutdown
               exit
+            elsif $mm_reload
+              $mm_reload = false
+              reload
+            end
           end
         end
       end
@@ -62,10 +67,18 @@ module Middleman
       def reload
         logger.info "== The Middleman is reloading"
 
-        unmount_instance
-        mount_instance
+        begin
+          app = new_app
+        rescue Exception => e
+          logger.error "Error reloading Middleman: #{e}\n#{e.backtrace.join("\n")}"
+          logger.info "== The Middleman is still running the application from before the error"
+          return
+        end
 
-        logger.info "== The Middleman is standing watch at http://#{host}:#{port}"
+        unmount_instance
+        mount_instance(app)
+
+        logger.info "== The Middleman has reloaded"
       end
 
       # Stop the current instance, exit Webrick
@@ -112,7 +125,8 @@ module Middleman
 
           # See if the changed file is config.rb or lib/*.rb
           if needs_to_reload?(added_and_modified + removed)
-            reload
+            $mm_reload = true
+            @webrick.stop
           else
             added_and_modified.each do |path|
               app.files.did_change(path)
@@ -168,8 +182,8 @@ module Middleman
       # Attach a new Middleman::Application instance
       # @param [Middleman::Application] app
       # @return [void]
-      def mount_instance
-        @app = new_app
+      def mount_instance(app)
+        @app = app
 
         @webrick ||= setup_webrick(@options[:debug] || false)