import_csv.rb and import_csv_spec.rb changed, CSVImporter::CSVImporterError class added

* reverted method name and all references from import_csv back to csv_importer
* added lib/csv_importer/csv_importer_errors to keep track of custom exceptions in CSVImporter module
* raised SkywardExtraneousFile from within import_skyward_students method, and rescued from within import method