#76 Set a message for error responses

Without one, clients might treat errors as generic failures instead of
handling them in a manner appropriate for their error code

src/main/java/io/kamax/mxisd/exception/InvalidPepperException.java

@@ -23,5 +23,6 @@ package io.kamax.mxisd.exception;
 public class InvalidPepperException extends RuntimeException {
 
     public InvalidPepperException() {
+        super("The provided pepper is invalid or expired");
     }
 }