Add MySQL datasource, refactor related classes to use this datasource

Includes:
* Moving the base query into the Mysql record source
* Allowing the error storage location to be overridden via ENV
Add the FilePathCorrecter class.

There was a feature of the CSV exporter that we needed in the MySQL
exporter - the ability to find a file location "logically" via a
cascading set of rules. This implements that logic when using a direct
MySQL data source.
Correct errors in the API docs.