archival: force a `CompressionError` to bytes before passing to `error.Abort`

Authored by mharbison72.


archival: force a CompressionError to bytes before passing to error.Abort

I'm not sure what changed before pytype 09-09-2021 (from 04-15-2021), but this
started getting flagged.

This fixes:

File "/mnt/c/Users/Matt/hg/mercurial/archival.py", line 199, in taropen: Function bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
         Expected: (self, ints: Iterable[int])
  Actually passed: (self, ints: tarfile.CompressionError)
  Attributes of protocol Iterable[int] are not implemented on tarfile.CompressionError: __iter__

Differential Revision: https://phab.mercurial-scm.org/D11472