ready-cache: Properly expose error source (#467)

The `ready_cache::error::Failed` type does not actually expose its inner
error type via `Error::source`. This change fixes this and improves
debug formatting.

This change adds a new constraint, ensuring that keys impl Debug in
order for the `error::Failed` type to impl Debug/Error.

tower/src/ready_cache/error.rs

@@ -6,9 +6,12 @@ pub struct Failed<K>(pub K, pub crate::BoxError);
 
 // === Failed ===
 
-impl<K> std::fmt::Debug for Failed<K> {
+impl<K: std::fmt::Debug> std::fmt::Debug for Failed<K> {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        std::fmt::Debug::fmt(&self.1, f)
+        f.debug_tuple("Failed")
+            .field(&self.0)
+            .field(&self.1)
+            .finish()
     }
 }
 
@@ -18,8 +21,8 @@ impl<K> std::fmt::Display for Failed<K> {
     }
 }
 
-impl<K> std::error::Error for Failed<K> {
+impl<K: std::fmt::Debug> std::error::Error for Failed<K> {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        self.1.source()
+        Some(&*self.1)
     }
 }