Potentially cancel all other submit handlers after my own
Ensure that your implementation of hook_form_alter() is the last one to be called. Your implementation of hook_form_alter() can then unshift one validation function and push a second validation function. The second validation function would then have complete information about whether other validation functions objected to saving the node (by calling form_get_errors()).

You can ensure that your your implementation of hook_form_alter() is the last one to be called by implementing hook_module_implements_alter()