Suggest re-running command with --trace on fail (#6551)

Merge pull request 6551

lib/jekyll/command.rb

@@ -73,6 +73,30 @@ module Jekyll
           "Fail if errors are present in front matter"
       end
       # rubocop:enable Metrics/MethodLength
+
+      # Run ::process method in a given set of Jekyll::Command subclasses and suggest
+      # re-running the associated command with --trace switch to obtain any additional
+      # information or backtrace regarding the encountered Exception.
+      #
+      # cmd     - the Jekyll::Command to be handled
+      # options - configuration overrides
+      # klass   - an array of Jekyll::Command subclasses associated with the command
+      #
+      # Note that all exceptions are rescued..
+      # rubocop: disable RescueException
+      def process_with_graceful_fail(cmd, options, *klass)
+        klass.each { |k| k.process(options) if k.respond_to?(:process) }
+      rescue Exception => e
+        raise e if cmd.trace
+
+        msg = " Please append `--trace` to the `#{cmd.name}` command "
+        dashes = "-" * msg.length
+        Jekyll.logger.error "", dashes
+        Jekyll.logger.error "Jekyll #{Jekyll::VERSION} ", msg
+        Jekyll.logger.error "", " for any additional information or backtrace. "
+        Jekyll.logger.abort_with "", dashes
+      end
+      # rubocop: enable RescueException
     end
   end
 end

lib/jekyll/commands/build.rb

@@ -15,7 +15,7 @@ module Jekyll
 
             c.action do |_, options|
               options["serving"] = false
-              Jekyll::Commands::Build.process(options)
+              process_with_graceful_fail(c, options, self)
             end
           end
         end

lib/jekyll/commands/serve.rb

@@ -72,25 +72,17 @@ module Jekyll
               opts["serving"] = true
               opts["watch"]   = true unless opts.key?("watch")
 
-              start(opts)
-            end
-          end
-        end
-
-        #
-
-        def start(opts)
               # Set the reactor to nil so any old reactor will be GCed.
-          # We can't unregister a hook so in testing when Serve.start is
-          # called multiple times we don't want to inadvertently keep using
-          # a reactor created by a previous test when our test might not
+              # We can't unregister a hook so while running tests we don't want to
+              # inadvertently keep using a reactor created by a previous test.
               @reload_reactor = nil
 
               config = configuration_from_options(opts)
-          if Jekyll.env == "development"
-            config["url"] = default_url(config)
+              config["url"] = default_url(config) if Jekyll.env == "development"
+
+              process_with_graceful_fail(cmd, config, Build, Serve)
+            end
           end
-          [Build, Serve].each { |klass| klass.process(config) }
         end
 
         #