Error out if key is missing

Author: marcan

pywii/Common/pywii/wii.py

@@ -1501,6 +1501,8 @@ def __init__(self, file, tmd, tik, certs, footer, nandwad=False):
         self.data_off = self.f.tell()
 
     def adddata(self, data, cid):
+        if self.tik.title_key is None:
+            raise Exception("Required key is not available")
         i = self.tmd.find_cr_by_cid(cid)
         cr = self.tmd.get_content_records()[i]
         cr.sha = SHA.new(data).digest()