Add the ability to validate a source

[yruslan]

Background

Currently exceptions thrown at sources's getRecordCount() is considered a validation error.
This is confusing and combines 2 concerns in one method.

Feature

Add validate() method to the Source interface

The validate method should allow returning errors as well as just warnings depending on the source configuration.

Proposed Solution

validate() method of the Source interface should have default implementation to return Ready. This will keep the interface backwards compatible.