XSD validation should not prevent project to build

Following NXS-6636, a validation error is thrown whenever the parsing of an XSD resource failed. The error could be because of a non-readable file but it could also be due to a missing import in the file that would be resolved when deployed on a Nuxeo Platform.

Blocking the build of the project is too drastic for validation errors that could be resolved at runtime during the deployment. We should throw a warning instead.