API errors during execution of BLE operations should be propagated in BLE response PDUs where possible and sensible

[ghost]

Right now if you write to LED Text to display a message on the LED display and the display is already busy, the request is ignored and there's an error returned by the micro:bit API call. But as far as the connected Bluetooth client is concerned, the write worked.

This specific issue may be dealt with in a different way (we may make the BLE write take priority and interrupt any current display animation) but it would be good if a general review of how API errors are handled in the context of Bluetooth operations was undertaken. Where possible and sensible, an error executing a BLE originated sequence of calls should result in an error code returned in the Bluetooth response PDU (if there is one).

[ghost]

Looks like mbed does not support this currently....

ARMmbed/ble#181