add fallible font methods to FontCollection

[acarl005]

Add more fallible getter methods a. la #62.

[acarl005]

Note that we have a Result<Option<FontFamily>, HRESULT> instead of just a Result<FontFamily, HRESULT>. I think this makes sense, as there may be no font family with the given name, so Ok(None) is returned to designate that the name doesn't exist, and no errors occurred.

An alternative I considered was creating a new error enum, e.g.

enum GetError {
    Win32Error(HRESULT),
    NotFound
}

And we could return Result<FontFamily, GetError> instead.

[mrobinson]

Suggested change

	// Find a font matching the given font descriptor in this font collection.
	/// Find a font matching the given font descriptor in this font collection.

[mrobinson]

@acarl005 I'm not able to modify your branch. Please apply the following diff and I will land this one:

diff --git a/src/font_collection.rs b/src/font_collection.rs
index 3e4974f..2154b28 100644
--- a/src/font_collection.rs
+++ b/src/font_collection.rs
@@ -114,6 +114,7 @@ impl FontCollection {
         self.font_family(index).unwrap()
     }
 
+    /// Returns the [`FontFamily`] at the given index.
     pub fn font_family(&self, index: u32) -> Result<FontFamily, HRESULT> {
         let mut family: *mut IDWriteFontFamily = ptr::null_mut();
         unsafe {
@@ -130,7 +131,7 @@ impl FontCollection {
         self.font_from_descriptor(desc).unwrap()
     }
 
-    // Find a font matching the given font descriptor in this font collection.
+    /// Find a font matching the given font descriptor in this [`FontCollection`].
     pub fn font_from_descriptor(&self, desc: &FontDescriptor) -> Result<Option<Font>, HRESULT> {
         if let Some(family) = self.font_family_by_name(&desc.family_name)? {
             let font = family.first_matching_font(desc.weight, desc.stretch, desc.style)?;
@@ -151,6 +152,7 @@ impl FontCollection {
         self.font_from_face(face).ok()
     }
 
+    /// Get a [`Font`] from the given [`FontFace`].
     pub fn font_from_face(&self, face: &FontFace) -> Result<Font, HRESULT> {
         let mut font: *mut IDWriteFont = ptr::null_mut();
         unsafe {
@@ -167,6 +169,8 @@ impl FontCollection {
         self.font_family_by_name(family_name).unwrap()
     }
 
+    /// Find a [`FontFamily`] with the given name. Returns `None` if no family
+    /// with that name is found.
     pub fn font_family_by_name(&self, family_name: &str) -> Result<Option<FontFamily>, HRESULT> {
         let mut index: u32 = 0;
         let mut exists: BOOL = FALSE;