windows: provide filename in IOError exceptions
This brings the Windows posixfile errors in line with the errors
on other platforms.
[hooks]
pretxncommit.cr = python:hgext.win32text.forbidcr
pretxnchangegroup.cr = python:hgext.win32text.forbidcr
Attempt to commit or push text file(s) using CR line endings
in dea860dc51ec: f
transaction abort!
rollback completed
abort: pretxncommit.cr hook failed
hello<LF>
hello<CR>