Remove panicking assert

[acarl005]

Hello, there are several assert!s in font_family::FontFamily, eg:

dwrote-rs/src/font_family.rs

Line 75 in 771b2e1

assert!(hr == 0);

Unfortunately, these are crashing our app. Can this crate provide fallible versions of these which return a Result instead so that we can recover from this error in a non-panicking way?

[acarl005]

I'm happy to make a PR if desired. Perhaps an analogous set of methods like try_get_font(&self, index: u32) -> Result<Font>, etc. can be added alongside the existing ones?

[jdm]

Making those methods fallible sounds reasonable to me!

[acarl005]

Awesome! I've submitted a PR please let me know what you think.