Fix invalid constant error reporting.

zentaro@ pointed out it's hard to debug syscall policy failures when
using an unrecognized constant.

Print an error when reading an invalid argument index, an invalid
constant, or an invalid errno value.

Bug: 32236891
Test: syscall_filter_unittest
Change-Id: Ifb20efcd9b107ca240ab37ac5e3ed501f55d6dea
3 files changed