clarify the unknown algorithm
diff --git a/otp.py b/otp.py
index f36bebc..83b6ea6 100755
--- a/otp.py
+++ b/otp.py
@@ -161,7 +161,7 @@
     pwd = pwds[seed]
 
   processor = ALGOS.get(algo)
-  assert processor, 'Unknown/unsupported algorithm.'
+  assert processor, 'Unknown/unsupported algorithm: "%s"' % (algo,)
 
   value = processor(seed + pwd, seq)
   response = ' '.join(to_words(value))