[supervisor-proc-exit-listener] Error message in syslog if feature is not supported

[SavchukRomanLv]

Description
Error message in syslog appears if feature is not supported
After changes, made in #4208 for nat container, error messages appears in syslog.

Steps to reproduce the issue:
Run crm test on t0 topology:
ansible-playbook test_sonic.yml -i inventory --limit <testbed> -e testbed_name=<testbed> -e testbed_type=t0 -e testcase_name=crm
After this changes also failed:
dhcp_relay
vxlan_decap
acl
interfaces
everflow_testbed
lldp
continuous_link_flap
pfc_wd
neighbour_mac_noptf
restart_swss
Describe the results you received:
Test fails due error in syslog:
ERR nat#supervisor-proc-exit-listener: Unable to retrieve features for container 'nat'. Exiting...
Describe the results you expected:

• all affected test cases passed
• If feature is not supported, has to be implemented some new logic (INFO of DEBUG message in syslog, not ERROR)

Additional information you deem important (e.g. issue happens only occasionally):
For nat service option autorestart was enabled in #4208. Error message appears here https://github.com/Azure/sonic-buildimage/blob/master/files/scripts/supervisor-proc-exit-listener lines 64-66.

**Output of `show version`:**

```
SONiC Software Version: SONiC.HEAD.591-dirty-20200309.044431
Distribution: Debian 9.12
Kernel: 4.9.0-11-2-amd64
Build commit: 590caaf5
Build date: Mon Mar  9 06:33:21 UTC 2020
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```