Fix assortment of errors Liz, uh, helpfully found
- make sure the error drop-down actually appears on screen when applicable in importing wizard (was appearing but not being displayed, requiring window re-size to show up)
- make carbon-14 calibration tolerant of sample ages with no error actually entered (and include helper function for future use-cases of the same nature)
- avoid breaking all iterations through all cores when a computation plan crashes (more hilarious __iter__ bugs)