Back end 500 when faceting on a quasi-categorical numeric variable

[bobular]

Seems to affect more than one study, and other visualizations (histogram at least)

The constraints allow numeric variables with few discrete values to be selected as facet variable, but they cause a 500.

[d-callan]

this was also reported in veupathdb/edadataservice#133, and i think can be qa'd as part of that issue. ill close this one.