Derive Debug for all error types (#28)

tower-balance/src/lib.rs

@@ -19,6 +19,7 @@ where T: Discover,
 }
 
 /// Error produced by `Balance`
+#[derive(Debug)]
 pub enum Error<T, U> {
     Inner(T),
     Balance(U),

tower-buffer/src/lib.rs

@@ -41,6 +41,7 @@ where T: Service,
 }
 
 /// Errors produced by `Buffer`.
+#[derive(Debug)]
 pub enum Error<T> {
     Inner(T),
     Closed,

tower-util/src/option.rs

@@ -18,6 +18,7 @@ pub struct ResponseFuture<T> {
 }
 
 /// Error produced by `OptionService` responding to a request.
+#[derive(Debug)]
 pub enum Error<T> {
     Inner(T),
     None,