Download and cache the IERS Bulletin A table.
If one is already cached, download a new one and overwrite the old. Store table in the astropy cache, and undo the monkey patching caused by earlier failure (if applicable).
If one does not exist, monkey patch
Timeobjects don’t raise errors by computing UT1-UTC off the end of the IERS table.
Trueshows a progress bar during the download.