In several cases, an error when processing the query would cause mysql to
return to the top level without printing warnings.  Fix is to always
print any available warnings before returning to the top level.


client/mysql.cc:
  In com_go(), ensure that warnings are printed even if an error
  occurred when processing the query.  Before this patch, an error
  in several places would return to the top level without printing
  associated warnings.
  
  Attempt to avoid printing the warning, though, if it's a duplicate
  of mysql_error() for the connection handle.
mysql-test/r/mysql.result:
  Add test for bug 25146
mysql-test/t/mysql.test:
  Add test for bug 25146