Rename exception_wrapper::getExceptionPtr to to_exception_ptr
Rename exception_wrapper::getExceptionPtr to to_exception_ptr

Summary:
[Folly] Rename `exception_wrapper::getExceptionPtr` to `to_exception_ptr`.

Make it clear that this will sometimes be an expensive operation. Using the word `to` evokes the possibility of an expensive conversion, while using the word `get` implies cheap access.

Reviewed By: djwatson

Differential Revision: D4391621

fbshipit-source-id: 33ca051d9be5a6050ba9f30b20faee35b7e58afb

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