Helper method to get exception type

Test Plan: Tested with subsequent thrift diff

Reviewed By: davejwatson@fb.com

FB internal diff: D1108428

folly/String.h

@@ -352,6 +352,13 @@ inline size_t demangle(const std::type_info& type, char* buf, size_t bufSize) {
   return demangle(type.name(), buf, bufSize);
 }
 
+/**
+ * Debug string for an exception: include type only.
+ */
+inline fbstring exceptionTypeStr(const std::exception& e) {
+  return folly::to<fbstring>(demangle(typeid(e)));
+}
+
 /**
  * Debug string for an exception: include type and what().
  */