Output cause of error when FileSystem.copy_entries method was failed.

- befor the change

    $ knife upload nodes/node.json
    ERROR: nodes/node.json failed to write: HTTP error writing: 400 "Bad Request"

- after the change

    $ knife upload nodes/node.json
    ERROR: nodes/node.json failed to write: HTTP error writing: 400 "Bad Request" cause: {"error":["Invalid key test in request body"]}
Change OperationFailedError itself to include the body in "message" if it's a 400.
Merge pull request #106 from k-yamada/master

Output cause of error when FileSystem.copy_entries method was failed.