[Share 2.0] Add exceptions to OCP

apps/files_sharing/api/share20ocs.php

apps/files_sharing/lib/controllers/sharecontroller.php

apps/files_sharing/tests/api/share20ocstest.php

apps/files_sharing/tests/controller/sharecontroller.php

lib/private/share20/defaultshareprovider.php

lib/private/share20/manager.php

lib/public/share/exceptions/genericshareexception.php

lib/public/share/exceptions/sharenotfound.php

tests/lib/share20/defaultshareprovidertest.php

tests/lib/share20/managertest.php