There were two distict bugs: parse error was returned for valid
statement and that error wasn't reported to the client.

The fix ensures that EXPLAIN SELECT..INTO is accepted by parser and any
other parse error will be reported to the client.


mysql-test/r/explain.result:
  Add result for bug#15463.
mysql-test/t/explain.test:
  Add test case for bug#15463.
sql/sql_parse.cc:
  Assert that if parsing error has occured then apropriate error message
  has been pushed into error stack.
sql/sql_yacc.yy:
  If there is no lex->result in select_var_ident rule, then we have
  to be in DESCRIBE mode.