Ignore 500 status code in generate plugin script

Due to various reasons, this script may encounter the
500 status code from some repo (x/fuel-plugin-onos in current case[1])
If that happen then it return failure status code to the
propose-updates job and fail that job

- https://zuul.openstack.org/builds?job_name=propose-updates&project=openstack%2Fdevstack&skip=0

It is better not to raise the 500 error in this script and just ignore those
repo to process further to detect the plugin.

[1] https://zuul.openstack.org/build/dba0aa41d145472397916dfcd13948de/log/job-output.txt#2442

Change-Id: Ibca0a2aac404161340e8fc00170018eecf5c8326
1 file changed