Use consistent return types/exceptions in method "get_files()"

[uniqueg]

Currently, get_files() returns way too many possible response types and raising of exceptions is inconsistent when setting or not setting format=zip. Moreover, the current behavior is not documented correctly.

Unmarshalled responses should only be returned if TRSClient is configured to run in debug mode (as documented). Otherwise, either an object of type trs_cli.models.error (if there's a non-200 response from the TRS), of type Path (if format=zip and all went well) or of type List[ToolFile] (if format=None and all went well) is to be returned. Exceptions (as currently documented) should also be returned consistently, regardless of what format is specified.