Friendlier error if filter is not found

If user sets, e.g., `filters [:name, :manager]`,
and manager is not set to `filterable true`,
RailsAdmin currently raises a NoMethodError
when trying to call `.type` on nil.

This change raises a more descriptive error
to help the user find the problem
Merge pull request #2079 from nathanl/friendlier_filter_error

Friendlier error if filter is not found