Bug#46958: Assertion in Diagnostics_area::set_ok_status, 
trigger, merge table
            
The problem with break statements is that they have very
local effects. Hence a break statement within the inner loop
of a nested-loops join caused execution to proceed to the
next table even though a serious error occurred. The problem
was fixed by breaking out the inner loop into its own
method. The change empowers all errors to terminate the
execution.
            
The errors that will now halt multi-DELETE execution
altogether are 
  - triggers returning errors
  - handler errors
  - server being killed


mysql-test/r/delete.result:
  Bug#46958: Test result.
mysql-test/t/delete.test:
  Bug#46958: Test case.
sql/sql_class.h:
  Bug#46958: New method declaration.
sql/sql_delete.cc:
  Bug#46958: New method implementation.