BugFix: Error propagation is silenced (#511)

* Properly propagate error of nested result

plotters/src/style/font/ttf.rs

@@ -137,7 +137,7 @@ fn load_font_data(face: FontFamily, style: FontStyle) -> FontResult<FontExt> {
                 .load()
                 .map(FontExt::new)
                 .map_err(|e| FontError::FontLoadError(Arc::new(e)))
-        })?;
+        })??;
     }
     drop(cache);