Consciously specify, and conscientiously apply, what so many projects leave to ad-hoc (mis)judgment: Develop a practical, consistent, and rational error handling policy early in design, and then stick to it. Ensure that it includes:
Identification: What conditions are errors.
Severity: How important or urgent each error is.
Detection: Which code is responsible for detecting the error.
Propagation: What mechanisms are used to report and propagate error notifications in each module.
Handling: What code is responsible for doing something about the error.
Reporting: How the error will be logged or users notified.
Change error handling mechanisms only on module boundaries.