Fail request when error occurs during download

[cesnietor]

fixes: #3006

Checks whether the download was complete in the frontend.
In the backend, setting the headers doesn't work if it fails during io.Copy cause it already starterd writing to the body. So we just log it and handle it in the client layer.

On error:
After:

Test Steps:

• Deploy a distributed minio e.g. using kind 4 nodes
• expose the service
• Upload a big object e.g. 100Mb
• Go to the Browser UI
• Cordon one of the nodes
• Download the object (enabling throttling in your browser to emulate slow speed)
• Delete one of the minio pods (it won't start since we cordoned a node)
• In the UI the object download should eventually fail (before it was downloading an inclomplete object)

/// Other tests

• Test downloading multiple files (selecting multiple)
• Test downloading multiple files (selecting one, then download, then only other one and click download) so that the download box appears.

[bexsoft]

Tested, LGTM

[vadmeste]

Removing the status code does not help debugging, can we try to add more information here ?

[cesnietor]

status by itself was also not very helpful. You'll need to see the logs for now. We were not exposing the errors before either.
Exposing detailed errors is in our backlog.