libbinder: don't ignore sendDecStrong error

I am not sure why this was ignored before, but I can't think of any
reason why it should be. Fail hard fail fast.

Note: this is being submitted independently in preparation for a series
of refcount changes related to performance.

Bug: 182940634
Test: binderRpcTest
Change-Id: Ifcc0bf96952a368c4ad5620b3f1d2d17feac40c5