folly/comprehensions - fixing exceptions

Summary:
need to open up the visibility of std::exception, otherwise
try..catch(const std::exception& e) won't work for these cases.

Test Plan: unit tests from D670355

Reviewed By: delong.j@fb.com

FB internal diff: D670633

folly/experimental/Gen.h

@@ -81,7 +81,7 @@ class GenImpl;
 template<class Self>
 class Operator;
 
-class EmptySequence : std::exception {
+class EmptySequence : public std::exception {
 public:
   virtual const char* what() const noexcept {
     return "This operation cannot be called on an empty sequence";