Tell us which attribute is missing from the resources.
Tell us which attribute is missing from the resources.

In https://github.com/ManageIQ/manageiq/pull/6962, we removed the guid column
from MiqGroup and it wasn't obvious why spec/requests/api/groups_spec.rb was
failing.

Adding the missing key/attribute should make this more obvious.

Before:

```
  Failure/Error: fetch_value(keys).each { |key| expect(results.all? { |r| r.key?(key) }).to be_truthy }
    expected: truthy value
        got: false
```

After:

```
  Failure/Error: fetch_value(keys).each { |key| expect(results.all? { |r| r.key?(key) }).to be_truthy, "resource missing: #{key}" }
    resource missing: guid
```

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