[Bug 33990] unhelpful error message on failure to decode s/mime encrypted message

[Andrew Jorgensen]

Public bug reported:
https://launchpad.net/malone/bugs/33990

Affects: evolution (Ubuntu)
       Severity: Normal
       Priority: (none set)
         Status: Unconfirmed

Description:
I got an s/mime encrypted message the other day and got the message

Decoder failed, error -8147

Took me some time before I realized that the problem was that I hadn't
imported my certificate.  I even installed Thunderbird in hopes that
this "decoder failure" might be a evolution-only problem.

The message should say something about what is actually wrong.