grep: convert an exception to bytes for a warning message
Caught by pytype:
File "/mnt/c/Users/Matt/hg/mercurial/commands.py", line 3457, in grep: Function _bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
Expected: (self, ints: Iterable[int])
Actually passed: (self, ints: re.error)
The following methods aren't implemented on re.error:
__iter__
Differential Revision: https://phab.mercurial-scm.org/D10173
(binary:application/octet-stream)