Improve error reporting for misconfiguration of Entities and Columns

[helrond]

Is your feature request related to a problem? Please describe.

Because the Fluxx API does not throw an explicit error message, there is currently no way to know if you’ve requested an Entity or Column that does not exist. This means it’s possible to add a field with a typo, but then that field would just be empty in the output

Describe the solution you'd like

Warn users when they’re requesting an entity or column which doesn’t exist.

Additional context

The need for this may be somewhat reduced by a spreadsheet import feature (#11)

[helrond]

It also looks like if the field value is null it is not returned, even if it references a correct key in the database.