addListener method
Registers a listener, which is a function to be called when new error information is set by ErrNotifier.set(), ErrNotifier.log(), LoggingErrNotifier.set() or LoggingErrNotifier.log().
This method must not be called after dispose() has been called.
Implementation
void addListener(ErrListener<T> listener) {
assert(_debugAssertNotDisposed());
_notifier.addListener(listener);
}