If a delayed insert thread was aborted by a concurrent 'truncate table'
statement, then the diagnostic area would fail with an assert in a debug build
because no actual error message was pushed on the stack despite a thread
being killed.
      
This patch adds an error message to the stack.


sql/sql_insert.cc:
  * Changed sql_print_error() to my_error() to avoid assertion in the DA
  * Added assertion in "should never happen" branch.