Log Message:
Changes 1.3.3:
Bug fixes
- Make ``SpiderLoader`` raise ``ImportError`` again by default for missing
  dependencies and wrong :setting:`SPIDER_MODULES`.
  These exceptions were silenced as warnings since 1.3.0.
  A new setting is introduced to toggle between warning or exception if needed ;
  see :setting:`SPIDER_LOADER_WARN_ONLY` for details.