when traffic dump directory is specified, now prints error message and exits instead of crashing when traffic file cannot be written
when traffic dump directory is specified, now prints error message and exits instead of crashing when traffic file cannot be written

Change-Id: I273e07aefd9faea13662f6699a6f3b074c8efe88

osmocom/osmo-tetra,osmocom/osmo-tetra,osmocom/osmo-tetra