Let ClientReceiveState::exceptionWrapper return const&
Let ClientReceiveState::exceptionWrapper return const&

Summary:
[Thrift] Let `ClientReceiveState::exceptionWrapper` return `const&`.

This will permit callers to avoid unnecessary atomic refcount ops arising from copying the stored `exception_ptr` or `shared_ptr` (or `runtime_error` or `logic_error` on libstdc++ >= 5), or any other allocations or other inefficiencies arising from copying arbitrary stored exceptions.

Reviewed By: eduardo-elizondo

Differential Revision: D5429038

fbshipit-source-id: ccd509a4caf381c04c186eb2f8c04704863868c4

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