do_action and apply_filter should report if the user function is missing

[nbachiyski]

do_action/apply_filters tell me somehow that I the user function doesn't exists. For example I have entered the action name wrong and there is no proper way to debug it other from luck and guessing.

Fixing do_action is easy -- it can just return false or WP_Error. The problem is with apply_filters -- it's return value is dependent on the user function and we can't interfere with it in no way.

[DD32]

You can call ​has_filter() to check if a function has been registered for a filter/action hook, Infact, calling that will require less code than checking for a error condition being returned.

[Viper007Bond]

Suggest wontfix. 99.9% of the time it doesn't matter if a filter/hook actually does anything or not. For those times that it does, has_filter() is a simple way to test.