Draft: Catch proxy errors

To be added..

Dont think I have time to check for the time left on a proxy though, at least not in this sprint :/

TODOL

• Check if the pipeline tester (liaison probably) has a proxy. If it doenst, fail with error message on the pipeline cf https://mattermost.web.cern.ch/lhcb/pl/15hqasgseir3udbg4b1xue4u1h

• CHeck if convener approver has a proxy. If it doesnt, fail with comment on gitlab cf lhcb-simulation/mc-requests!502 (merged)