nRF5xn::init don't verify if errors have occurred during btle_init

[pan-]

See this at: https://github.com/ARMmbed/ble-nrf51822/blob/master/source/nRF5xn.cpp#L94 .
The init process should check and report if something has gone wrong

[rainierwolfcastle]

ARM Internal Ref: IOTSFW-1168

[andresag01]

@pan- @rgrover : It is easy to simply return BLE_ERROR_UNSPECIFIED when a call to ble_init() fails. However, this might not be the ideal solution. The main problem is that we do not have sufficient error codes to be more specific about the cause of the problem. Is it acceptable to return BLE_ERROR_UNSPECIFIED when there is a failed call to ble_init()? Should we are more error codes to ble/blecommon.h in BLE API?

[pan-]

What would you want to return ?

[andresag01]

Submitted a pull request ARMmbed/ble#155 to add another error code to the API