Bug #192341 - When the chflags command does not exit successfully,

Bug #192341 - When the chflags command does not exit successfully,
try to generate an informative error. First, use stat or lstat to
try and generate an ENOENT error.  It the path exists, verify that
the chflags binary exists and raise CommandNotFound if necessary.
Finally, simply generate an EPERM OSError with the output of the
command since we're not sure exactly why it failed or what the
real errno was.

svn path=/main/trunk/; revision=7834