Take StringPiece as argument to get_or_throw
Take StringPiece as argument to get_or_throw

Summary: Avoids eager allocation of std::string when it's not necessary

Reviewed By: yfeldblum

Differential Revision: D37764759

fbshipit-source-id: 7866aa9883ecde194b45b7998e8527abf44fb844

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly