Add handling of msgs with no blankline before body

Add try...except to catch ValueErrors raised when there's no blank line
beween header and body of an otherwise valid signed message. Raising a
CryptoException means that the message will be rejected nicely in
Ssm2.on_message.

ssm/crypto.py

@@ -160,7 +160,10 @@ def verify(signed_text, capath, check_crl):
 
     # SMIME header and message body are separated by a blank line
     lines = message.strip().splitlines()
-    blankline = lines.index('')
+    try:
+        blankline = lines.index('')
+    except ValueError:
+        raise CryptoException('No blank line between message header and body')
     headers = '\n'.join(lines[:blankline])
     body = '\n'.join(lines[blankline + 1:])
     # two possible encodings