WebBasket: migrate to exception logging

• Migrates error logging to new exception-based technique. (addresses #580)
• Creates class 'InvenioWebBasketWarning' for warnings.
• Deletes var CFG_WEBBASKET_WARNING_MESSAGES.
• Deletes warnings var whenever it is needed.
• Modifies 'tmpl_warnings'.
• Updates the comments of functions.
• Adds warnings in 'perform_request_create_basket'.
• Removes warning ERR_WEBBASKET_NOT_OWNER not defined.
• Adds 'req' parameter in calls to 'register_exception' whenever it is available.
• Adds calls to 'gettext_set_language' to translate the exception messages.
• Merge notes:
  • Fixed uninitialised variable cases with warnings_rights.
  • Fixed return tuple in perform_request_display_public().