Add cancellation support to coroutine interface

Reviewed By: andriigrynenko

Differential Revision: D17749079

fbshipit-source-id: 44c2c6dd55fe5be033bc4a5139941b5e3a004a67

folly/CancellationToken.h

@@ -28,6 +28,11 @@ namespace folly {
 
 class CancellationCallback;
 class CancellationSource;
+struct OperationCancelled : public std::exception {
+  const char* what() const noexcept override {
+    return "coroutine operation cancelled";
+  }
+};
 
 namespace detail {
 class CancellationState;