Expound on HIDL Return errors.

Add 'onValueRetrieval' error which explicitly talks about the error
being while retrieving a value (since 'assertOk' was publically
exposed, it no longer makes sense to have a specific error).

assertOk is now also used in ~return_status and contains even more
information about how to interpret the error.

Bug: many
Test: TH

Change-Id: I80a7ec2ae42b7980a0a84fc760d5b27b17f293db
2 files changed