Return a warning if one of the items contains an error

[e13h]

https://github.com/evanphilipsmith/gsheets-plaid/blob/89ec87c4efb686b790adf84399f9a7ebf1838fe0/gsheets_plaid/web_server/main.py#L313-L318

The GetItemRequest will still return successfully if the item is in an error state. Perhaps instead of returning a boolean we could return an enum state (one of GOOD, ERROR, or WARNING)?