On error log STDOUT if STDERR is empty

If a process doesn't correctly log to STDERR then the default .run! log
message isn't very helpful.  This logs command_result.error if it is
present, otherwise it logs command_result.output.

lib/awesome_spawn.rb

@@ -100,8 +100,10 @@ def run!(command, options = {})
 
     if command_result.failure?
       message = CommandResultError.default_message(command, command_result.exit_status)
+      error_message = command_result.error.present? ? command_result.error : command_result.output
+
       logger.error("AwesomeSpawn: #{message}")
-      logger.error("AwesomeSpawn: #{command_result.error}")
+      logger.error("AwesomeSpawn: #{error_message}")
       raise CommandResultError.new(message, command_result)
     end