Catch missing trigger_duration when launching check vectors for external events; fixes #311
Some GRB experiments (AGILE and possibly INTEGRAL, their documentation is unclear) don't include integration times in their GCN notices, leading to this value being missing in the external event and a failure when this value is used: https://sentry.io/organizations/ligo-caltech/issues/1531937668/
We should instead catch this and use a default value if missing (4 sec chosen here).