Fixed how unexpected Option errors were being worded

src/error/unexpected.rs

@@ -105,16 +105,19 @@ impl<T, E: error::Error> Mappable<T> for result::Result<T, E> {
     }
 }
 
-static OPTION_NONE_ERROR: &str = "expected Some but got None";
+#[derive(thiserror::Error, Debug)]
+enum OptionError {
+    #[error("required but not given")]
+    None,
+}
 
 impl<T> Mappable<T> for Option<T> {
     fn or_unexpected(self) -> Result<T> {
-        self.ok_or(Error::from(OPTION_NONE_ERROR)).or_unexpected()
+        self.ok_or(OptionError::None).or_unexpected()
     }
 
     fn or_unexpected_while<D: fmt::Display>(self, prefix: D) -> Result<T> {
-        self.ok_or(Error::from(OPTION_NONE_ERROR))
-            .or_unexpected_while(prefix)
+        self.ok_or(OptionError::None).or_unexpected_while(prefix)
     }
 
     fn map_unexpected_while<F, D>(self, f: F) -> Result<T>
@@ -122,8 +125,7 @@ impl<T> Mappable<T> for Option<T> {
         F: FnOnce() -> D,
         D: fmt::Display,
     {
-        self.ok_or(Error::from(OPTION_NONE_ERROR))
-            .map_unexpected_while(f)
+        self.ok_or(OptionError::None).map_unexpected_while(f)
     }
 }