failure warning for base36 parsing

src/lib.rs

@@ -71,7 +71,7 @@ impl AnimationFrames for Vec<Image> {
 }
 
 fn from_base36(str: &str) -> u64 {
-    u64::from_str_radix(str, 36).unwrap()
+    u64::from_str_radix(str, 36).expect(&format!("Invalid base36 string: {}", str))
 }
 
 /// this doesn't work inside ToBase36 for some reason