mosquitto_pub: exit if server becomes unreachable and QOS > 0

Fixes eclipse/mosquitto#1899

In stdin line mode, mosquitto_pub will continue running and accepting
input even if/when/after publishing fails.  This condition is reached
when it first successfully establishes a connection and the server later
is unreachable.

Exiting with a non-zero exit code allows for much easier health
monitoring when used in a long-running pipe.

Signed-off-by: Dan White <dan.white@valpo.edu>