add(subscription): SubscriptionErr::into_inner

this allows not leaking the SubscriptionErr type anywhere more, but it
is sort of kicking the can on the way as we are starting to desperately
need proper error types instead of the stringified one.

Author: koivunej

src/subscription.rs

@@ -219,6 +219,16 @@ impl<E: Debug + PartialEq> fmt::Display for SubscriptionErr<E> {
 
 impl<E: Debug + PartialEq> std::error::Error for SubscriptionErr<E> {}
 
+impl<E: Debug + PartialEq> SubscriptionErr<E> {
+    pub fn into_inner(self) -> Option<E> {
+        use SubscriptionErr::*;
+        match self {
+            Cancelled => None,
+            Failed(e) => Some(e),
+        }
+    }
+}
+
 /// Represents a request for a resource at different stages of its lifetime.
 pub enum Subscription<T, E> {
     /// A finished `Subscription` containing the desired `TRes` value.