extensions: catch OSError when hgext is not accessible (
issue1708)
Temporary workaround for
issue1708: on win32 with py2exe, hgext is distributed
inside a zipped file (which anyway does not contain the py files from which we
ought to extract the documentation strings), which raises a WindowsError
(subclasses OSError).
This means that on such platforms the list of disabled extensions won't be
available. Real fix postponed for after Mercurial 1.3.