[PATCH] Fixes a crasher in RTF style export

[David Mandelin]

The ctor of _rtf_font_info crashed if any exported style lacked a
font-family property. A few notes:

- Ctor now throws exception if the style has no font-family.
- The exception is a class with no members. I doubt this is standard C++
(and the web copy of the standard I use is down right now), but I
couldn't think of anything useful to throw. Suggestions welcome.