lib/httpserver: add Unwrap method to ErrorWithStatusCode, so As and Is functions in standard errors package may properly unwrap the error inside ErrorWithStatusCode

lib/httpserver/httpserver.go

@@ -455,6 +455,13 @@ type ErrorWithStatusCode struct {
 	StatusCode int
 }
 
+// Unwrap returns e.Err.
+//
+// This is used by standard errors package. See https://golang.org/pkg/errors
+func (e *ErrorWithStatusCode) Unwrap() error {
+	return e.Err
+}
+
 // Error implements error interface.
 func (e *ErrorWithStatusCode) Error() string {
 	return e.Err.Error()