Improve CEL message

[lsviben]

Description of your changes

Improves the message for toplevel CEL rules, noting the full path of the field.
Also fixes the issue where initProvider would be mentioned in the CEL message due to it
not being set.

Fixes #273

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• [] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Tested with the example from #273 .

Initial result:

The EventHub "example-eventhub" is invalid:
* spec: Invalid value: "object": no such key: initProvider evaluating rule: messageRetention is a required parameter
* spec: Invalid value: "object": no such key: initProvider evaluating rule: partitionCount is a required parameter

After applying the change:

The EventHub "example-eventhub" is invalid: 
* spec: Invalid value: "object": spec.forPovider.messageRetention is a required parameter
* spec: Invalid value: "object": spec.forPovider.partitionCount is a required parameter

[turkenh]

Thanks for the quick fix @lsviben 🙌