Stop worrying notice when encrypted email not signed

When no signature is present, there was a message saying
"Unable to determine validity of cryptographic signature".
This fix means that if there are no error messages and no
messages about signature verification then the message is
assumed to not be signed at all. This fix also saves the
encrypted messages to a temp file with a suffix of .asc
to stop gpg complaining about "unknown suffix".

Hamish Downer