sql/share/errmsg-utf8.txt:
  correct the error message, as query_cache_type variable is not read-ony anymore
sql/sql_cache.cc:
  the caller should verify that query cache resize
  is possible, before trying it
sql/sys_vars.cc:
  * test if qc resize is possible in the sysvar on_check() funntion,
    not in the on_update() function.
  * use the error message that better describes the problem