RPC Binder: clearer errors for wrong transact type

Bug: 423127919 # this bug error is totally different
 than this case. If we hit this case, we would have
 gotten a different harder error. However, I found
 this while looking at this bug.
Test: binderRpcTest
Flag: EXEMPT bug fix
Change-Id: I3e0db452ebeed1a520ffc0181ed3577eb38bea9a