[OpenMP][libomptarget][NFC] Remove error data member from AsyncInfoWrapperTy

This patch removes the Err data member from the AsyncInfoWrapperTy class. Now the error
is stored externally, in the caller side, and it is explicitly passed to the
AsyncInfoWrapperTy::finalize() function as a reference.

Differential Revision: https://reviews.llvm.org/D148027