* Added unit-tests for treeWalkPool
* Added unit tests for tree-walk-fs
* Added period at the end of all comments.
* FS/XL: Unified tree walk tests for both backends
* Added disk failure related tests for treewalk
Replaced removeRandomDisks with removeDiskN. There is no need to
randomize disks that fail while the distribution of chunks in XL during
erasure coding data is random.
This function is added to extend the standard error responses.
Which is needed in some cases for example CompleteMultipartUpload
should respond with ErrPartTooSmall error when parts uploaded are
lesser than 5MB (i.e minimum allowed size per part).
Fixes#1536
The following code crashes when upload ID does not
exist, since we are setting err == nil when we find
err == errFileNotFound.
```
if e == nil {
t.Fatal(e.Error())
```
Fix it.
