Same as in_pce_thread/1, but wait for Goal to be completed.
Success depends on the success of executing Goal. If Goal throws
an exception, this exception is re-thrown by in_pce_thread/1.
Possible bindings of Goal are returned, but be aware that the
term has been copied. If in_pce_thread_sync/1 is called in the
thread running pce, it behaves as once/1.