Better detection of encryption method needed

[Lewis Rosenthal]

If we are presented with an encryption type which we cannot handle, e.g., certificate, we need to present a more useful dialog or notification to the user. Presently, we present the Password dialog anytime we encounter an encrypted document, even if that document cannot be opened (unsupported) or uses a certificate (likewise currently unsupported, but in any case would never require a password).

I'm not sure how much detail Poppler returns beyond RC4, AES, and AES-256, so we may not yet be able to tell what encryption is being used.

Related: #301