Fix infinite recursion on unknown Level.String()

Using `%q` in the error string causes `MarshalText` to recurse (by calling String()).

logrus.go

@@ -74,7 +74,7 @@ func (level Level) MarshalText() ([]byte, error) {
 		return []byte("panic"), nil
 	}
 
-	return nil, fmt.Errorf("not a valid lorus level %q", level)
+	return nil, fmt.Errorf("not a valid logrus level %d", level)
 }
 
 // A constant exposing all logging levels