RDB Loader: enrich error with table name

[BenFradet]

It'd be cool to add some context to the error reported when loading data, e.g.:

Data loading error Amazon Invalid operation: Number of jsonpaths and the number of columns should match. JSONPath size: 12, Number of columns in table or column list: 13

it would useful to get the table name

[alexanderdean]

Unfortunately this rather uninformative message is generated by the Redshift driver.

But, the whole problem should go away when we get rid of JSON Paths files.