fwk_io: Replace `fwk_assert` for error handling
Created by: leandro-arm
fwk_assert
calls were removed to minimize assertions in the framework code, since they can be easily replaced by error handling. The reasons to do that are that the framework should be reliable enough, the debugging process of the framework should not rely on the users and the error handling can facilitate the usage of the framework.
Change-Id: Ib9c71580cb0c489b50a63d9fb5957ecf9459d212