Add strerror equivalent in libprivilege for more informative error logging.

Add strerror equivalent in libprivilege for more informative error logging.

[Issue#]      SSDWSSP-646
[Bug/Feature] Add strerror equivalent in libprivilege for more informative
              error logging.
[Cause]       Currently there is no way to check the reason for the
              error apart from looking directly into the code.
[Solution]    An strerror equivalent was created to convert error code
              to human-readable error description.
[Verfication] Build, install, run tests.

Change-Id: I9cd4416133e782d52cf9a8488e7b10fcf82546f2