[cloud_firestore] Better error message for permission denied

[nerder]

The error message for when there is a permission denied is not helpful in understanding which rule is causing the problem.

[cloud_firestore/permission-denied] The caller does not have permission to execute the specified operation.. Error thrown null.

It would be better to know which is the actual operation the user was trying to attempt (ie: read users operation, update users)
I'm not sure if this is possible to do, but in this way is very hard to actually understand and fix the problem, especially when it happens only to a really tiny portion of the user base in our use case.