Add stricter error checking to unittests

Explicity check for expected stderr strings for failure cases.
This will ensure we hit the expected error without hitting any additional
errors along the way.

Changed to AIDL_ERROR instead of LOG(ERROR) for a few cases to give uniform
error messages.

Test: runtests.sh
Change-Id: I4acc5beb940694684193080183c8cfe19e3e8071
3 files changed