Error message when collection is missing is not clear

[edalzell]

Bug Description

If you have an entries fieldtype referencing a non-existent collection and you reference the fieldset in the CP, the error is not very clear.

How to Reproduce

Make a fieldset (by hand) that has an entries fieldtype w/ a non-existent collection:

                    field:
                      type: entries
                      max_items: 1
                      width: 50
                      display: Entry
                      collections:
                        - i-dont-exist
                        - blog

Extra Detail

https://flareapp.io/share/o7Aeqd7p

Environment

Statamic version: 3.0.0-beta-27

PHP version: 7.4

Install method (choose one):

• Fresh install from statamic/statamic (with migrator)

[duncanmcclean]

I've replicated this bug and created a fix (#2422)