Improve handling of non existing relation data

[Wouter van Bommel]

When hooks fire slowly, there can be situation in which the controllers already have removed relation data, which hasn't been processed by all charms.

If charms then try to process the relation data, they get reported an `ERROR permission denied` which is pretty confusion.

We can reproduce this by directly calling `relation-get`, which valid (or invalid) options like:

juju run -u external-services/0 -- relation-get -r db:1951 --app - snaprevs-r7b37bcf
ERROR permission denied

&

juju run -u external-services/0 -- relation-get -r db:1931 --app - snaprevs-r7b37bcf
{}

Where in example 1, a response that would indicate something like `NOT FOUND` would improve the debugging experience.