push-to-pull-request: Fix shell usage in an error path.
push-to-pull-request: Fix shell usage in an error path.

Errors go to stderr, not stdout.

While we're here, match the message itself to the others in
this script.

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