For nested sql errno handlers (unlike sqlexception and other), we didn't stop
  searching when the innermost handler was found - now make sure we do.


mysql-test/r/sp.result:
  Updated result for BUG#15011.
mysql-test/t/sp.test:
  New testcase for BUG#15011.
sql/sp_rcontext.cc:
  Make sure we stop at the innermost sql_errno handler.