Replace check_log_err with log_includes/log_excludes

The latter are more explicit and easier to understand at the call site.

Also removed the redundant crash checks via the log files.

maxscale-system-test/bug729.cpp

@@ -79,7 +79,7 @@ int main(int argc, char* argv[])
     Test->tprintf("Executing PHP script: %s\n", str);
     Test->add_result(system(str), "PHP script FAILED!\n");
 
-    Test->check_log_err(0, (char*) "Can't route MYSQL_COM_STMT_PREPARE", false);
+    Test->log_excludes(0, "Can't route MYSQL_COM_STMT_PREPARE");
 
     int rval = Test->global_result;
     delete Test;