[Libomptarget] Check errors when synchronizing the async queue

Summary:
Currently when we synchronize the asynchronous queue for the plugins, we
ignore the return value. This is problematic because we will continue on
like nothing happened if the kernel fails.

Fixes https://github.com/llvm/llvm-project/issues/60814

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D144191