A single invalid relation should not block all others

[johnsca]

If a single relation has incompatible or missing versions, it blows up during construction of the SDI class. While these conditions should be surfaced, they should not block accessing the relations which are functional.

[knkski]

We have to be pretty careful here. Blowing up on any bad relation when constructing a SerializedDataInterface instance has one useful property: it can't be accidentally ignored.

In thinking about what SerializedDataInterface.get_data would look like with some broken relations, I think my response is similar to my response over here:

https://github.com/canonical/serialized-data-interface/pull/32/files#discussion_r764548160

If you want fine-grained control, why not just iterate in the charm over self.model.relations[relation_name] yourself? get_data is just a convenience wrapper for doing that combined with enforced schema checking.