Add error checking to payload['previous'] for Circle.CI.
Add error checking to payload['previous'] for Circle.CI.

This webhook used to fail if there was no previous payload.

kou/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,jrowan/zulip,zulip/zulip,rishig/zulip,kou/zulip,jrowan/zulip,eeshangarg/zulip,jackrzhang/zulip,mahim97/zulip,tommyip/zulip,shubhamdhama/zulip,vaidap/zulip,synicalsyntax/zulip,hackerkid/zulip,jackrzhang/zulip,verma-varsha/zulip,dhcrzf/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,punchagan/zulip,vabs22/zulip,punchagan/zulip,vaidap/zulip,shubhamdhama/zulip,vabs22/zulip,verma-varsha/zulip,mahim97/zulip,amanharitsh123/zulip,amanharitsh123/zulip,brainwane/zulip,jackrzhang/zulip,hackerkid/zulip,kou/zulip,Galexrt/zulip,dhcrzf/zulip,andersk/zulip,tommyip/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,synicalsyntax/zulip,jackrzhang/zulip,mahim97/zulip,hackerkid/zulip,Galexrt/zulip,showell/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,punchagan/zulip,andersk/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,verma-varsha/zulip,timabbott/zulip,eeshangarg/zulip,Galexrt/zulip,jackrzhang/zulip,rishig/zulip,eeshangarg/zulip,hackerkid/zulip,vabs22/zulip,vabs22/zulip,showell/zulip,tommyip/zulip,amanharitsh123/zulip,vabs22/zulip,tommyip/zulip,brainwane/zulip,Galexrt/zulip,synicalsyntax/zulip,vaidap/zulip,zulip/zulip,showell/zulip,kou/zulip,andersk/zulip,timabbott/zulip,dhcrzf/zulip,synicalsyntax/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,jrowan/zulip,Galexrt/zulip,brainwane/zulip,vaidap/zulip,zulip/zulip,amanharitsh123/zulip,mahim97/zulip,synicalsyntax/zulip,brockwhittaker/zulip,timabbott/zulip,synicalsyntax/zulip,mahim97/zulip,brainwane/zulip,dhcrzf/zulip,brainwane/zulip,verma-varsha/zulip,brockwhittaker/zulip,jrowan/zulip,brockwhittaker/zulip,rishig/zulip,shubhamdhama/zulip,showell/zulip,brockwhittaker/zulip,shubhamdhama/zulip,timabbott/zulip,jackrzhang/zulip,hackerkid/zulip,amanharitsh123/zulip,tommyip/zulip,dhcrzf/zulip,verma-varsha/zulip,brainwane/zulip,rishig/zulip,zulip/zulip,jrowan/zulip,tommyip/zulip,timabbott/zulip,brockwhittaker/zulip,kou/zulip,vaidap/zulip,rishig/zulip,vaidap/zulip,amanharitsh123/zulip,Galexrt/zulip,kou/zulip,mahim97/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,verma-varsha/zulip,rishig/zulip,tommyip/zulip,punchagan/zulip,rht/zulip,kou/zulip,Galexrt/zulip,showell/zulip,jackrzhang/zulip,jrowan/zulip,andersk/zulip,dhcrzf/zulip,vabs22/zulip,brockwhittaker/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip,rishig/zulip,andersk/zulip,rht/zulip,dhcrzf/zulip