Using On Error

I am having several problems with my project which involves three or more applications each one doing certain things depending on what the other applications do.

I have to test this project in the run-mode (I can't test in the IDE because it requires the same application to run multiple instances of itself)

I want to set up On Error statements in all subs and functions. I want to make sure that when any sub or function exits the On Error for that sub or function is no longer in affect. In the error handler part I want to describe the error, error number, and the sub or function name the error occured in. How to do this.