Fix: return with error code when mqtt connect fails

[Kraemii]

What:
Handle error of mqtt_connect
SC-566

Why:
When mqtt_connect failed, no error was returned and openvas thought the MQTT connection was successful even when it was not.

How:
Start a scan without a broker. Compare before patch and after. Before yout get these messages:

libgvm util:CRITICAL:2022-03-23 09h26.41 utc:4416: mqtt_connect: mqtt connection error to tcp://127.0.0.1:1883: Failure
sd   main:MESSAGE:2022-03-23 09h26.41 utc:4416: attack_network_init: INIT MQTT: SUCCESS 

after the patch you get

sd   main:MESSAGE:2022-03-23 10h34.47 utc:21180: Running LSC via Notus for 127.0.0.1
libgvm util:CRITICAL:2022-03-23 10h34.47 utc:21180: mqtt_connect: mqtt connection error to tcp://127.0.0.1:1883: Failure
libgvm util:WARNING:2022-03-23 10h34.47 utc:21180: mqtt_init: Unable to connect to MQTT broker.

Checklist:

• Tests
• PR merge commit message adjusted