Add tests for SignupWorker error handling.
Add tests for SignupWorker error handling.

This brings SignupWorker coverage to 100%.

synicalsyntax/zulip,rishig/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,zulip/zulip,zulip/zulip,tommyip/zulip,eeshangarg/zulip,jackrzhang/zulip,brainwane/zulip,rishig/zulip,synicalsyntax/zulip,punchagan/zulip,timabbott/zulip,timabbott/zulip,andersk/zulip,rishig/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,shubhamdhama/zulip,shubhamdhama/zulip,rht/zulip,rishig/zulip,shubhamdhama/zulip,dhcrzf/zulip,punchagan/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,zulip/zulip,brainwane/zulip,tommyip/zulip,jackrzhang/zulip,dhcrzf/zulip,hackerkid/zulip,dhcrzf/zulip,rht/zulip,dhcrzf/zulip,eeshangarg/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,andersk/zulip,rht/zulip,tommyip/zulip,dhcrzf/zulip,punchagan/zulip,dhcrzf/zulip,hackerkid/zulip,rht/zulip,kou/zulip,showell/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,jackrzhang/zulip,tommyip/zulip,timabbott/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,kou/zulip,showell/zulip,brainwane/zulip,zulip/zulip,tommyip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,synicalsyntax/zulip,andersk/zulip,jackrzhang/zulip,shubhamdhama/zulip,jackrzhang/zulip,showell/zulip,rishig/zulip,showell/zulip,timabbott/zulip,jackrzhang/zulip,punchagan/zulip,kou/zulip,showell/zulip,showell/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,timabbott/zulip,zulip/zulip,punchagan/zulip,dhcrzf/zulip,punchagan/zulip,jackrzhang/zulip,andersk/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,brainwane/zulip,andersk/zulip,brainwane/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,tommyip/zulip