Fixes the following bug:

- Bug #32125: Database crash due to ha_innodb.cc:3896: ulint convert_search_mode_to_innobase

  When unknown find_flag is encountered in convert_search_mode_to_innobase()
  do not call assert(0); instead queue a MySQL error using my_error() and
  return the error code PAGE_CUR_UNSUPP. Change the functions that call
  convert_search_mode_to_innobase() to handle that error code by "canceling"
  execution and returning appropriate error code further upstream.