Bluetooth: host: Improve error feedback on invalid input

Improve the error feedback when calling bt_le_adv_start and inputting
too much data in the advertisement.

Error feedback before:

Bluetooth initialized
Advertising failed to start (err -22)

Error feedback after:

Bluetooth initialized
[bt] [ERR] set_ad: Advertising data does not fit in buffer
Advertising failed to start (err -22)

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>