Fix compiler warning (mimblewimble#70)

src/lib.rs

@@ -482,7 +482,7 @@ pub enum Error {
 // Passthrough Debug to Display, since errors should be user-visible
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        f.write_str(error::Error::description(self))
+        f.write_str(&self.to_string())
     }
 }