set exception message for InvalidNullError

Author: Charlie Somerville

lib/graphql/invalid_null_error.rb

@@ -5,13 +5,12 @@ class InvalidNullError < GraphQL::Error
     def initialize(field_name, value)
       @field_name = field_name
       @value = value
+      super("Cannot return null for non-nullable field #{@field_name}")
     end
 
     # @return [Hash] An entry for the response's "errors" key
     def to_h
-      {
-        "message" => "Cannot return null for non-nullable field #{@field_name}"
-      }
+      { "message" => message }
     end
 
     # @return [Boolean] Whether the null in question was caused by another error