Handle disconnects when accessing memory

[krichardsson]

This PR adds fixes to detect and handle radio disconnects while accessing the mapped memory.
If the Crazyflie is disconnected/turned off the failed callback will now be called to let the application know that the read/write failed. A future improvement could be to also add a reason for the failure.

Fixes #366

[ataffanel]

Good addition, every call with a callback should contain a fail callback if the main callback is not guaranteed to be called in case of error (similar to promises in JS ...). Good to add one to the memory subsystem.