check-openapi: Improve error handling.
check-openapi: Improve error handling.

Previously a YAML syntax error resulted in an
UnhandledPromiseRejectionWarning and a successful exit code; now it
gives a clear message and a failing exit code.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

kou/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,zulip/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,zulip/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,showell/zulip,shubhamdhama/zulip,showell/zulip,andersk/zulip,timabbott/zulip,eeshangarg/zulip,rht/zulip,timabbott/zulip,zulip/zulip,showell/zulip,timabbott/zulip,synicalsyntax/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,brainwane/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,shubhamdhama/zulip,synicalsyntax/zulip,synicalsyntax/zulip,showell/zulip,kou/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,zulip/zulip