Consider warning/error handling like GCC's -Werror

[chrispcampbell]

In #284 we added better error reporting for cases where a CSV cell contains a non-numeric value, but decided to leave it as a soft error. There are cases where one might prefer soft error / warning vs hard error / fail-fast. In some cases you want them treated like warnings (and continue processing) so that you can get a sense of how many issues there are. For production builds, you probably want them treated like hard errors (and fail fast).

We have a couple other issues (e.g., #158) where SDE doesn't fail fast though it probably should.

I think we could cover both use cases by adding a flag that works like GCC's -Werror, i.e., treat things as a warning by default (just log the error), but allow for treating them as a hard error if the flag is passed on the command line.