This is needed as explained by @krisis
Lets say we have following errors.
```
[]error{nil, errFileNotFound, errDiskAccessDenied, errDiskAccesDenied}
```
Since the last two errors are filtered, the maximum is nil,
depending on map order.
Let's say we get nil from reduceErr. Clearly at this point
we don't have quorum nodes agreeing about the data and since
GetObject only requires N/2 (Read quorum) and isDiskQuorum
would have returned true. This is problematic and can lead to
undersiable consequences.
Fixes#3298
This change is needed to make reading from objects future proof
in-terms of handling online disks. Our current counter is not
based on affirmative knowledge and relies on arithmetic sequence
which can lead to bugs.
Using modTime simplifies the understanding of `xl.json` and future
tooling / debugging of the format.
Harshavardhana