When logging an exn from cpp2 processing, include the type
When logging an exn from cpp2 processing, include the type

Summary: [Thrift] When logging an exn from cpp2 processing, include the type by using `folly::exceptionStr`.

Reviewed By: stevegury

Differential Revision: D13846847

fbshipit-source-id: 4e22bfa037b162bb2564884469fbb3f17cf9c277

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