Check survey files before validating an Audience against them

Reads the survey files the way a validation reads them, together as one survey, and says whether they hold questions and published results to compare against, before any validation starts. The first call starts the reading and answers reading; repeat the same call until done (the reading takes seconds for a short file and a few minutes for a long deck). failure.code is SURVEY_RESULTS_NOT_FOUND for a questionnaire without results, SURVEY_NOT_FOUND when no survey questions were found, or SURVEY_FILES_UNREADABLE; null when the files can be validated against. A validation started with the same files reuses the reading. data is null when files cannot be checked ahead; the validation then reads them itself. Nothing is spent.