Add middleware.NextOrFailure (#462)

This checks if the next middleware to be called is nil, and if so returns
ServerFailure and an error. This makes the next calling more robust and
saves some lines of code.

Also prefix the error with the name of the middleware to aid in
debugging.

middleware/pkg/debug/debug.go

@@ -2,6 +2,7 @@ package debug
 
 import "strings"
 
+// Name is the domain prefix we check for when it is a debug query.
 const Name = "o-o.debug."
 
 // IsDebug checks if name is a debugging name, i.e. starts with o-o.debug.