Don't print inner error when returning as source

According to rust-lang/project-error-handling#44 (comment), we should not be printing the inner error AND returning it as source. Libraries like `anyhow` will traverse the chain of `source` errors and build up a composed error message. Printing it and returning the same error from `source` results in duplicate error messages in that case.

core/src/transport.rs

@@ -42,6 +42,7 @@ pub mod upgrade;
 mod boxed;
 mod optional;
 
+pub use self::boxed::boxed;
 pub use self::boxed::Boxed;
 pub use self::choice::OrTransport;
 pub use self::memory::MemoryTransport;
@@ -393,7 +394,7 @@ where
             TransportError::MultiaddrNotSupported(addr) => {
                 write!(f, "Multiaddr is not supported: {}", addr)
             }
-            TransportError::Other(err) => write!(f, "{}", err),
+            TransportError::Other(_) => Ok(()),
         }
     }
 }