libbinder_rs: Modify asserts to propagate failure to C++

We can't rely on Rust panics to unwind, so we need to return failed
transaction rather than panic on assertion failures. Replaces assert!
and assert_eq! to propagate failures as failed transactions rather than
Rust panics.

Test: atest rustBinderSerializationTest
Bug: 179067202
Change-Id: I01b6443b364a9df2a5631f2ddfbc72ce13febe93