mailpipe: don't raise UnverifiedSignatureMessage for valid signatures.

Before this patch, signatures like:

  ... signature
    summary:
      CRL missing: False
      CRL too old: False
      bad policy: False
      green: True
      key expired: False
      key missing: False
      key revoked: False
      red: False
      signature expired: False
      system error: False
      valid: True
    ...

would raise the exception.  Now they won't.