Modified files.rb to handle tar error

Changed the run_pipeline! function to instead use Open3.pipeline_start
In doing so, the code is able to grab the last STDERR. In the case
where the error is the one from older tar versions (relating to ./),
it will not raise a Backup::Error. All other instances of command
failure will result in Backup::Error being raised.

lib/backup/files.rb

@@ -71,8 +71,18 @@ module Backup
     end
 
     def run_pipeline!(cmd_list, options = {})
-      status_list = Open3.pipeline(*cmd_list, options)
-      raise Backup::Error, 'Backup failed' unless status_list.compact.all?(&:success?)
+      err_r, err_w = IO.pipe
+      options[:err] = err_w
+      status = []
+      Open3.pipeline_start(*cmd_list, options) do |threads|
+        err_w.close
+        threads.collect { |t| status.push(t.value) }
+      end
+      unless status.compact.all?(&:success?)
+        unless err_r.read =~ /^g?tar: \.: Cannot mkdir: No such file or directory/
+          raise Backup::Error, 'Backup failed'
+        end
+      end
     end
   end
 end