plugin-api: deviced: Elaborate error code

plugin-api: deviced: Elaborate error code

Plugin api has changed to return negative error value in three ways.
 1. -ENOTSUP
    : Fail to load plugin backend.

 2. -EOPNOTSUPP
    : Successfully loaded plugin backend, but there is no implementation
      of requested interface.

 3. The other negative values
    : Successfully loaded plugin backend, and successfully invoked
      implemented interface, but the implementation itself returned
      a negative value. In this case, it is strongly discouraged
      that implementation returning -ENOTSUP or -EOPNOTSUPP as it
      cannot be distinguished from the above two cases.

Change-Id: I754c06a813c7c4587c07bc285677fdc1a99cbc80
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>