Using Enum

If you want to raise your own specific errors and have code to handle these errors you could put everything into a dedicated class.
Define all your errors using an Enum.

Use the vbObjectError built-in constant to ensure the custom error codes do not overlap with reserved/existing error codes.
The Err.Raise method can also take custom Description and Source parameters so it worth having a description associated with each error.

Create a dedicated private method to raise each error:

To raise the error, just call the correspnding subroutine.