Enable HTML validation warnings as failures

Most problems that tidy detects appears to be classed as warnings, not
errors, as HTML5 validation rules can be quite lax.

This turns on warnings in our validation results just to see how many
we actually get.

tests/integration/web/crawler_test.py

@@ -46,7 +46,7 @@
     'doctype': 'auto',
     'output_xhtml': True,
     'input_encoding': 'utf8',
-    'show-warnings': False,
+    'show-warnings': True,
 }
 
 TIDY_IGNORE = [