Fix handling of missing pack index file

Fix handling of missing pack index file

As demonstrated in
`UploadPackHandleDeletedPackFile.testV2IdxFileRemovedDuringUploadPack`
the fetch operation will fail when the pack index file is removed.

This is due to a wrapping of `FileNotFoundException` (which is a
subclass of `IOExeption`) in an `IOException` at PackIndex L#68. This
is then changing the behaviour of error handling in
`Pack.file.getBitmapIndex()` where the `FileNotFoundException` is
swallowed and allows the fetch process to continue. With FNFE being
wrapped in IOE, this blows up and breaks the fetch operation.

Simply rethrowing `FileNotFoundException` from `PackFile.open()` fixes
the broken fetch operation. This will also mark the whole pack as
invalid in the `IOException` handler in `Pack.idx()` method.

Change-Id: Ibe321aa1af21d26500e1cb2eb3464cc99a6dbc62
Signed-off-by: Dariusz Luksza <dariusz.luksza@gmail.com>