add exception

Author: Lena

draft-yun-cfrg-arc.md

@@ -1517,6 +1517,9 @@ Parameters:
 - generatorG: Element, equivalent to G.GeneratorG()
 - generatorH: Element, equivalent to G.GeneratorH()
 
+Exceptions:
+- NumberTooBigError, raised when v is out of range
+
 if G.ScalarToInt(v) >= upper_bound:
     raise NumberTooBigError