Improve error handling when aquatone-gather is run on system without X11

Nightmarejs will fail to process pages when run on a system without a
graphical desktop session and would make aquatone-gather fail all pages
processings with a rather obscure `JSON::ParserError` exception.

If JSON fails to parse the Nightmarejs driver will raise
`Aquatone::Browser::Drivers::IncompatabilityError` exception which tells
the user to run aquatone in a system with a graphical desktop session.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
  - Capture potential `NameError` exception in `asked_for_progress?` method,
    related to known JRuby bug (issue #4)
  - Capture potential `Errno::EBADF` exception in `asked_for_progress?` method (issue #15)
+ - Improve handling of error when aquatone-gather is run on a system without a graphical desktop session (X11)
 
 
 ## [0.1.1]

lib/aquatone/browser/drivers/nightmare.rb

@@ -1,6 +1,9 @@
 module Aquatone
   class Browser
     module Drivers
+
+      class IncompatabilityError < StandardError; end
+
       class Nightmare
         attr_reader :url, :vhost, :html_destination, :headers_destination, :screenshot_destination, :options
 
@@ -23,6 +26,8 @@ def visit
           wout.close
           command_output = rout.readlines.join("\n").strip
           JSON.parse(command_output)
+        rescue JSON::ParserError
+          fail IncompatabilityError, "Nightmarejs must be run on a system with a graphical desktop session (X11)"
         rescue => e
           process.stop if process
           return {

lib/aquatone/commands/gather.rb

@@ -78,17 +78,23 @@ def process_pages
         @tasks.shuffle.each do |task|
           host, port, domain = task
           pool.schedule do
-            output_progress if asked_for_progress?
-            visit = visit_page(host, port, domain)
-            if visit['success']
-              output("#{green('Processed:')} #{Aquatone::UrlMaker.make(host, port)} (#{domain}) - #{visit['status']}\n")
-              @successful += 1
-            else
-              output("   #{red('Failed:')} #{Aquatone::UrlMaker.make(host, port)} (#{domain}) - #{visit['error']} #{visit['details']}\n")
-              @failed += 1
+            begin
+              output_progress if asked_for_progress?
+              visit = visit_page(host, port, domain)
+              if visit['success']
+                output("#{green('Processed:')} #{Aquatone::UrlMaker.make(host, port)} (#{domain}) - #{visit['status']}\n")
+                @successful += 1
+              else
+                output("   #{red('Failed:')} #{Aquatone::UrlMaker.make(host, port)} (#{domain}) - #{visit['error']} #{visit['details']}\n")
+                @failed += 1
+              end
+              jitter_sleep
+              @task_count += 1
+            rescue Aquatone::Browser::Drivers::IncompatabilityError => e
+              output("\n")
+              output(red("Incompatability Error:") + " #{e.message}\n")
+              exit 1
             end
-            jitter_sleep
-            @task_count += 1
           end
         end
         pool.shutdown