Consistency with folly::SemiFuture continuations 5/n: make deferError use the same exception unpacking machinery as thenError

Summary:
Step 5 in adding full set of r-value-qualified unambiguous continuation methods to folly::Future for consistency with folly::SemiFuture.
 * Back point explicit exceptions election to deferError.

Reviewed By: marshallcline

Differential Revision: D8124828

fbshipit-source-id: 123b1cf460d2df4c8f359eff7329d34fbd8e0a9b