Handle undecodable characters in description

We've historically expected that the return values from libmagic are
ascii, since they are constant strings or stuff like dates/numbers.
In some cases, however, it will return information like the title of
the document in the doc's native character set, which is unknown to
us.  This produces decode errors.

I have not been able to get a document that triggers this behavior,
but the safest change is to decode with 'backslashreplace', which
escapes undecodable characters with a backslash.