Multiple bugfixes and code cleanup

[hexdigest]

Hi,

This PR solves multiple bugs:

• HTTP dereferencer tried to validate loaded schema with itself which is incorrect. Document referenced with JSON Pointer is not necessarily a JSON Schema. There should be a check that after all links are dereferenced we get valid JSON Schema.
• There was a problem to reference a root of the document (without "#")

More verbosity added to errors messages and logs.

[xino12]

Thanks for the contribution @hexdigest !