Fix regression bug where broken link warnings weren't displayed

lib/gluegun.rb

@@ -72,6 +72,8 @@ def self.gluegun_generate_pages(site_config_file)
                 end
                 begin
                   File.open(File.join(dest_path,"/#{key2["Slug"]}.html"), "w+") do |f|
+                    # Check if link is valid
+                    open(link)
                     partial_erb_arr.each do |partial_erb|
                       # Set nil to "-" to activate "<%-" and "-%>" characters
                       # for non-printing lines in erb file.
@@ -108,8 +110,10 @@ def self.gluegun_generate_pages(site_config_file)
 
     def self.reveal(link)
       begin
-        response = GitHub::Markdown.render_gfm(open(link).read)
-        return response
+        if (!link.nil?)
+          response = GitHub::Markdown.render_gfm(open(link).read)
+          return response
+        end
       rescue OpenURI::HTTPError
       end
     end