pull in LS::Error into std::exception hierarchy

:Release Notes:
Allow usual catching of exception with referencing to what() method

:Detailed Notes:
- implemented std::exception in LS::Error

:Testing Performed:
unit-tests on desktop

:QA Notes:

:Issues Addressed:
[BHV-1589] Implement category registration with its description

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: Ide392ecbef15c45814e7189293dc7e601e5f3d2d
Reviewed-on: https://g2g.palm.com/5254
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>