This is due to the fact that ReadAt() will not send io.EOF
when reading past the offset, server instead returns
Range error. Handle the error by only reading until the
requested size.
This fixes#1889.
```sh
mc mirror -w
```
Now runs continous even when the source is not available.
Reconnects indefinitely in a binomially increasing fashion
and also allows jitter so that the retry has a nice
distribution as well.
This allows for 'mc mirror -w' to run continously without
further monitoring.
Previously it was failing like below.
```
$ mc ls --incomplete myminio/mybucket
[2016-09-16 09:48:57 PDT] 576MiB 1.iso
$ mc ls --incomplete myminio/mybucket/1.iso
mc: <ERROR> Unable to initialize target ‘myminio/mybucket/1.iso’. Object key is missing, object key cannot be empty
```
This patch fixes this issue.
