Add exception handling to rake guides:validate

If the validation of a guide encounters an exception, inform about it
and continue with the loop to validate the rest.

guides/w3c_validator.rb

@@ -38,7 +38,12 @@ module RailsGuides
       errors_on_guides = {}
 
       guides_to_validate.each do |f|
-        results = validator.validate_file(f)
+        begin
+          results = validator.validate_file(f)
+        rescue Exception => e
+          puts "\nCould not validate #{f} because of #{e}"
+          next
+        end
 
         if results.validity
           print "."