WS-179: subtask #6 -- remove argparse namespace dependencies in classes

This addresses https://open-jira.nrao.edu/browse/WS-184.

• args are accessed only in locations_report.init()
• directory/file-related errors throw custom exception

apps/cli/executables/datafetcher/datafetcher/file_retrievers.py

@@ -52,7 +52,7 @@ class NGASFileRetriever:
         download_url = "http://" + server + "/RETRIEVE"
         destination = self._get_destination(file_spec)
         if destination.exists() and not self.force_overwrite and not self.dry_run:
-            raise FileExistsError(f"{destination} exists; aborting")
+            raise FileErrorException(f"{destination} exists; aborting")
 
         self._make_basedir(destination)