[libc++][PSTL] Overhaul exceptions handling

This makes exception handling a lot simpler, since we don't have to convert any exceptions this way. Is also properly handles all the user-thrown exceptions.

Reviewed By: ldionne, #libc

Spies: arichardson, mstorsjo, libcxx-commits

Differential Revision: https://reviews.llvm.org/D154238