Unhelpful error message for bulk upload

[gsrohde]

When the trait_covariate_associations table contains references to non-existent trait ids (CORRECTION: variable ids?; this table only refers to the variables table; see UPDATE below), the bulk upload will fail with the message "undefined method `name' for nil:NilClass", which is not very helpful.

The best solution is to ensure this error never occurs by imposing foreign-key constraints.

UPDATE: There already are foreign-key constraints on this table. Not sure what the cause was of the error I was seeing.