svesh

I tested this on 0.2.15. This bug remained in the new release

Logs: ``` panic: packFileIntoTar: failed to open file '/var/postgres/master_db/base/16401/1973388674.73' : open /var/postgres/master_db/base/16401/1973388674.73: no such file or directory goroutine 104881 [running]: github.com/wal-g/wal-g/internal.(*Bundle).handleTar.func1(0xc000172000, 0xc0013482d0, 0x30, 0x1c08ee0, 0xc00547c0d0, 0xc00251ec40, 0x0, 0x1c08b80, 0xc004a88cc0) /home/travis/gopath/src/github.com/wal-g/wal-g/internal/bundle.go:383...