update docs, test invalid x509 version
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 28da8ec..b86d3ee 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -43,3 +43,8 @@
 
     This is raised when the verify method of a one time password function's
     computed token does not match the expected token.
+
+
+.. class:: InvalidX509Version
+
+    This is raised when an X.509 certificate has an invalid version number.