Drop TX if RX is on-going and a signal is detected

This patch polls the modem status register before sending a message: if a message is currently being received the outgoing message is dropped and an appropriate error enum is returned. Additionally, if the buffer happens to be too small for the outgoing message, another error is returned.

Note that the radio must be set to receive after a longfi_send so that the status register is reset.

While this patch may should lower the chance of collision, it does not protect against it.