Handle Discovery API no results

[aliciatb]

If no results for a given combination of selections, return friendly message stating no assets of that type exist rather than Error: $ operator is invalid for atomic vectors or Error: no applicable method for 'unnest_tokens_' applied to an object of class "character" or Error: no applicable method for 'group_by_' applied to an object of class "character" or Error: no applicable method for 'mutate_' applied to an object of class "character".