Watcher: more structured error handling, phase out `DynError`

[ok300]

The error types in nexus-watcher are a combination of DbError, EventProcessorError, RunError, DynError.

A good first step towards a more useful structure is to replace DynError with clearer types, like variants of EventProcessorError or DbError.

[ok300]

Not having stronger error types leads to fragile downcast patterns in some places:

pubky-nexus/nexus-watcher/src/service/processor.rs

Lines 167 to 173 in bebbe25

	fn extract_retry_event_info(event: &Event, error: DynError) -> Option<(String, RetryEvent)> {
	let retry_event = match error.downcast_ref::<EventProcessorError>() {
	Some(EventProcessorError::InvalidEventLine { message }) => {
	error!("{}", message);
	return None;
	}
	Some(event_processor_error) => RetryEvent::new(event_processor_error.clone()),