Initialize _file in MetaStoreReader before open

So that __del__() will work properly if open fails.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
save-cmd.py: don't test for meta index via access()

Instead just try to open the metadata index and catch any EACCES.
There may be an issue with access on Cygwin
(http://bugs.python.org/issue2528), and even if not, this approach
removes a potential race between access() and open().

Thanks to Mark J Hewitt <mjh@idnet.com> for reporting the problem and
helping track down the cause.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>