Perform loader-specific pre-save validation before opening a file

[zml2008]

Currently, loaders are passed an opened Writer before they have any chance to validate that the provided node is a valid format, and do their validation in the write() method. This means that if validation fails, the target file will be erased.

We can add another extension point to AbstractConfiguration loader to perform pre-processing, which will allow us to preserve existing data, even when trying to write invalid data.