Add a folly::exceptionStr overload

Summary: For std::exception_ptr.

Test Plan: .

Reviewed By: davejwatson@fb.com

FB internal diff: D600345

folly/String.h

@@ -17,6 +17,7 @@
 #ifndef FOLLY_BASE_STRING_H_
 #define FOLLY_BASE_STRING_H_
 
+#include <exception>
 #include <string>
 #include <boost/type_traits.hpp>
 
@@ -285,6 +286,16 @@ inline fbstring exceptionStr(const std::exception& e) {
   return folly::to<fbstring>(demangle(typeid(e)), ": ", e.what());
 }
 
+inline fbstring exceptionStr(std::exception_ptr ep) {
+  try {
+    std::rethrow_exception(ep);
+  } catch (const std::exception& e) {
+    return exceptionStr(e);
+  } catch (...) {
+    return "<unknown exception>";
+  }
+}
+
 /*
  * Split a string into a list of tokens by delimiter.
  *