eldbus: Check return value and error out if necessary

eldbus: Check return value and error out if necessary

Coverity reports that we do not check the return value of
_eldbus_model_proxy_load here. We likely should be checking that in
case the model proxy fails to load so we can return proper values here.

Fixes Coverity CID1355233

@fix

Signed-off-by: Chris Michael <cp.michael@samsung.com>