Implement a listPart wrapper for checking progress
Provide an error message if upload ID is not specified
Implement a pause method

This method only sets a flag. Actual pause needs to be implemented in
_upload_part.
Some TODO notes to self
Provide an error message if resume_from is invalid
Do not try read ETag from response header

This won't be required if we list all parts and read corresponding ETags
in _complete_upload.
Do not try to rewrite upload_id during resume

res is undefined during resumption. User is required to set upload_id befo-
re trying to resume a paused upload.
Stop uploading if _pause is set
List all parts and use the ETags for upload completion request

This also automatically aborts dangling uploads without a single completed
part. Such cases are better restarted from scratch.
Return an error message if the last part upload has failed