billing: Add nocoverage for some untested error cases.
billing: Add nocoverage for some untested error cases.

Ideally, we'd have tests covering these cases.

rht/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,rht/zulip,punchagan/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,kou/zulip