obscure error when specification id not UUID v4

[matt-beanland]

Working on diffo_example, we encountered a situation where instances were not being created, and the failure was failure to pattern match on :ok in Feature.set_features_argument. Turns out the root cause was the specification uuid's not being v4.

[matt-beanland]

build_before returns a changeset, but we should use Ash.Changeset.add_error(changeset, error) to add the error. Each method should do this, build_before would be unchanged.

We should also probably have a verifier in the Provider extension that verifies that the specification id is valid uuid v4.

[matt-beanland]

Diffo.Provider.Specification requires id to validate as Diffo.Util.uuid4?, however this was checking UUID.info version ==4 but not variant == :rfc4122, which we added.

We also changed Specification.set_specified_by_argument to add the error to the changeset as per the Ash Error Handling Guide rather than simply logging it.

[matt-beanland]

There are other places where we manipulate the changeset and should add an error. Feature.set_features_argument and Characteristic.set_characteristics_arguments could both error, most likely if the characteristic value type is unknown at runtime. Improved these, had to use a rescue as struct() could raise UndefinedFunctionError.