Add the first draft of an event handler system

src/errors.rs

@@ -1,4 +1,5 @@
 use crate::entry::Entry;
+use crate::event_handlers::EventHandler;
 use std::fmt;
 
 pub struct Error {
@@ -41,3 +42,9 @@ impl From<std::sync::mpsc::SendError<Entry>> for Error {
         Error::new(&format!("IO error: {}", error))
     }
 }
+
+impl From<std::sync::mpsc::SendError<EventHandler>> for Error {
+    fn from(error: std::sync::mpsc::SendError<EventHandler>) -> Self {
+        Error::new(&format!("IO error: {}", error))
+    }
+}