Align Java & .NET validators

[ewoutkramer]

We should align the validators on:

1. Error codes
2. Settings
3. Interface used to resolve resources

Ad 1. This makes it easier to write unit-tests for both platforms
Ad 3. Maybe align IResourceResolver with the Conformance Service specs (as yet to be written in R4)

[wardweistra]

Perhaps this one can be closed in favor of #1262

[mmsmits]

Will be split into multiple issues, marked with label "align-with-java"