Add return code to UART TX function to be able to detect transmit
errors and propagate this information to higher layers.

UART RX callback functions return status codes, but they were never
checked in the library code. Handle failures of UART RC callbacks
and propagate them to higher layers.

This prevents infinite blocking when BT chip is not present or is not
responding to the commands. Usually, functions check for pending data
using UART peek callback first, but in some cases data is just awaited
(sl_bt_wait_response, sli_bgapi_get_event), causing infinite blocking
when BT chip is not present or not responding.