Added TryFire

This allows handlers to return errors which will be returned to the caller.
Also replaced error returned from On with a panic.

errors.go

@@ -0,0 +1,16 @@
+package events
+
+import "fmt"
+
+type HandlerError struct {
+	topic string
+	errs  []error
+}
+
+func (h HandlerError) Error() string {
+	return fmt.Sprintf("caught %d errors from topic '%v': %v", len(h.errs), h.topic, h.errs)
+}
+
+func (h HandlerError) Unwrap() []error {
+	return h.errs
+}