ActivitySim runs upstream models when spec file defined poorly

[JoeJimFlood]

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If a coefficient referenced in a spec file isn't defined in the corresponding coefficient file, it runs though the entire model flow and only crashes when it gets to that model step. For example, I ran placeholder_sandag_1_zone and added a coefficient called coef_nonexistent to the trip mode choice spec file. As shown in the log file for that run, it ran through all of the upstream model steps. This means that the error won't be caught for hours when running a full scale model. Would it be useful to add something that checks for issues such as this one at the start of a run and then stops the run immediately as opposed to waiting a few hours? That way modelers could fix it immediately without potentially wasting a day if they start a run right before finishing work in the evening. I already have a potential starting point in a branch of my fork of the repo.