Add an error code to signify an application error

[oscarbailey-xmos]

https://github.com/xmos/lib_device_control/blob/master/lib_device_control/api/control.h#L21
Currently, the control_ret_values enum only contains error messages related to the transport layer. If an application fails, it must return a "CONTROL_SUCCESS" code even if the execution of the command may have failed for an application-specific reason.

PR to implement this: #63