Document invalid_key class.

Signed-off-by: Michał Zientkiewicz <[email protected]>

include/dali/core/error_handling.h

@@ -88,6 +88,15 @@ struct unsupported_exception : std::runtime_error {
   explicit unsupported_exception(const std::string &str) : runtime_error(str) {}
 };
 
+/** An exception thrown when an invalid dictionary key is provided
+ *
+ * The exception denotes an invalid key. It can be thrown when:
+ * - the key is not found and the function returns a non-nullable type
+ * - the key doesn't meet some constraints (e.g. a dictionary doesn't accept an empty
+ *   string as a key).
+ *
+ * This exception is used at the Python boundary to raise KeyError rather than IndexError.
+ */
 struct invalid_key : std::out_of_range {
   explicit invalid_key(const std::string &message) : std::out_of_range(message) {}
   explicit invalid_key(const char *message) : std::out_of_range(message) {}